About "Fun with numbers"

Fun with numbers :

Mathematics is a subject with full of fun, magic and wonders. In this unit, we are going to enjoy with some of this fun and wonder.

Numbers in general form

Let us take the number 42 and write it as

42 = 40 + 2 = 10 × 4 + 2

Similarly, the number 27 can be written as

27 = 20 + 7 = 10 × 2 + 7

In general, any two digit number ab made of digits ‘a’ and ‘b’ can be written as

ab = 10 × a + b = 10a + b

ba = 10 × b + a = 10b + a

Now let us consider the number 351.

This is a three digit number. It can also be written as

351 = 300 + 50 + 1 = 100 × 3 + 10 × 5 + 1 × 1

In general, a 3-digit number abc made up of digit a, b and c is written as

abc  =  100 x a + 10 x b + 1 x c

=  100a + 10b + 1c

In the same way, the three digit numbers cab and bca can be written as

cab= 100c + 10a + b

bca = 100b + 10c + a

Games with Numbers

Reversing the digits of a two digit number

Anderson asks David to think of a 2 digit number, and then to do whatever he asks him to do, to that number.

Their conversation is shown in the following figure. 

Study the figure carefully before reading on.

Conversation between Anderson and David :

Now let us see if we can explain Anderson's  “trick”. Suppose, David chooses the number ab, which is a short form for the 2 -digit number 10a + b. On reversing the digits, he gets the number ba = 10b + a.

When he adds the two numbers he gets :

(10a + b) + (10b + a)  =  11a + 11b

= 11(a + b)

So the sum is always a multiple of 11, just as Anderson had claimed.

Dividing the answer by 11, we get (a + b)

(i.e.) Simply adding the two digit number.

Identify the pattern and find the next three terms

Study the pattern in the sequence.

(i) 3, 9, 15, 21, (Each term is 6 more than the term before it)

If this pattern continues, then the next terms are ___ , ___ and ___ .

(ii) 100, 96, 92, 88, ___ , ___ , ___ . (Each term is 4 less than the previous term )

(iii) 7, 14, 21, 28, ___ , ___ , ___ . (Multiples of 7)

(iv) 1000, 500, 250, ___ , ___,___. (Each term is half of the previous term)

(v) 1, 4, 9, 16, ___ , ___ , ___ . (Squares of the Natural numbers)

Number patterns in Pascal’s Triangle

The triangular shaped, pattern of numbers given below is called Pascal’s Triangle.

Activity : 

Identify the number pattern in Pascal’s triangle and complete the 6th row.

3 × 3 Magic Square

Look at the above table of numbers. This is called a 3 x 3 magic square. In a magic square, the sum of the numbers in each row, each column, and along each diagonal is the same.

In this magic square, the magic sum is 27. Look at the middle number. The magic sum is 3 times the middle number. Once 9 is filled in the center, there are eight boxes to be filled. Four of them will be below 9 and four of them above it.

They could be,

(a) 5, 6, 7, 8 and 10,11,12,13 with a difference of 1 between each number.

(b) 1, 3, 5, 7 and 11,13,15,17 with a difference of 2 between them or it can be any set of numbers with equal differences such as - 11,- 6,- 1, 4 and 14,19, 24, 29 with a difference of 5.

Once we have decided on the set of numbers, say 1, 3, 5, 7 and 11,13,15,17 draw four projections out side the square, as shown in the figures given below and enter the numbers in order, as shown in a diagonal pattern.

The number from each of the projected box is transferred to the empty box on the opposite side.

The Revolving Number

1 4 2 8 5 7

First set out the digits in a circle as given below. 

Now multiply 142857 by the number from 1 to 6 as given below.

We observe that the number starts revolving the same digits in different combinations. These numbers are arrived at starting from a different point on the circle.

After having gone through the stuff given above, we hope that the students would have understood "Fun with numbers". 

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Variables and constants

Writing and evaluating expressions

Solving linear equations using elimination method

Solving linear equations using substitution method

Solving linear equations using cross multiplication method

Solving one step equations

Solving quadratic equations by factoring

Solving quadratic equations by quadratic formula

Solving quadratic equations by completing square

Nature of the roots of a quadratic equations

Sum and product of the roots of a quadratic equations 

Algebraic identities

Solving absolute value equations 

Solving Absolute value inequalities

Graphing absolute value equations  

Combining like terms

Square root of polynomials 

HCF and LCM 

Remainder theorem

Synthetic division

Logarithmic problems

Simplifying radical expression

Comparing surds

Simplifying logarithmic expressions

Negative exponents rules

Scientific notations

Exponents and power


Quantitative aptitude

Multiplication tricks


Aptitude test online


Test - I

Test - II


Horizontal translation

Vertical translation

Reflection through x -axis

Reflection through y -axis

Horizontal expansion and compression

Vertical  expansion and compression

Rotation transformation

Geometry transformation

Translation transformation

Dilation transformation matrix

Transformations using matrices





Converting customary units worksheet

Converting metric units worksheet

Decimal representation worksheets

Double facts worksheets

Missing addend worksheets

Mensuration worksheets

Geometry worksheets

Comparing  rates worksheet

Customary units worksheet

Metric units worksheet

Complementary and supplementary worksheet

Complementary and supplementary word problems worksheet

Area and perimeter worksheets

Sum of the angles in a triangle is 180 degree worksheet

Types of angles worksheet

Properties of parallelogram worksheet

Proving triangle congruence worksheet

Special line segments in triangles worksheet

Proving trigonometric identities worksheet

Properties of triangle worksheet

Estimating percent worksheets

Quadratic equations word problems worksheet

Integers and absolute value worksheets

Decimal place value worksheets

Distributive property of multiplication worksheet - I

Distributive property of multiplication worksheet - II

Writing and evaluating expressions worksheet

Nature of the roots of a quadratic equation worksheets

Determine if the relationship is proportional worksheet



Trigonometric ratio table

Problems on trigonometric ratios

Trigonometric ratios of some specific angles

ASTC formula

All silver tea cups

All students take calculus 

All sin tan cos rule

Trigonometric ratios of some negative angles

Trigonometric ratios of 90 degree minus theta

Trigonometric ratios of 90 degree plus theta

Trigonometric ratios of 180 degree plus theta

Trigonometric ratios of 180 degree minus theta

Trigonometric ratios of 180 degree plus theta

Trigonometric ratios of 270 degree minus theta

Trigonometric ratios of 270 degree plus theta

Trigonometric ratios of angles greater than or equal to 360 degree

Trigonometric ratios of complementary angles

Trigonometric ratios of supplementary angles 

Trigonometric identities 

Problems on trigonometric identities 

Trigonometry heights and distances

Domain and range of trigonometric functions 

Domain and range of inverse  trigonometric functions

Solving word problems in trigonometry

Pythagorean theorem


Mensuration formulas

Area and perimeter



Types of angles 

Types of triangles

Properties of triangle

Sum of the angle in a triangle is 180 degree

Properties of parallelogram

Construction of triangles - I 

Construction of triangles - II

Construction of triangles - III

Construction of angles - I 

Construction of angles - II

Construction angle bisector

Construction of perpendicular

Construction of perpendicular bisector

Geometry dictionary

Geometry questions 

Angle bisector theorem

Basic proportionality theorem


Analytical geometry formulas

Distance between two points

Different forms equations of straight lines

Point of intersection

Slope of the line 

Perpendicular distance


Area of triangle

Area of quadrilateral



Matrix Calculators

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Chemistry periodic calculator


Missing addend 

Double facts 

Doubles word problems


Direct proportion and inverse proportion

Constant of proportionality 

Unitary method direct variation

Unitary method inverse variation

Unitary method time and work


Order of rotational symmetry

Order of rotational symmetry of a circle

Order of rotational symmetry of a square

Lines of symmetry


Converting metric units

Converting customary units


HCF and LCM  word problems

Word problems on simple equations 

Word problems on linear equations 

Word problems on quadratic equations

Algebra word problems

Word problems on trains

Area and perimeter word problems

Word problems on direct variation and inverse variation 

Word problems on unit price

Word problems on unit rate 

Word problems on comparing rates

Converting customary units word problems 

Converting metric units word problems

Word problems on simple interest

Word problems on compound interest

Word problems on types of angles 

Complementary and supplementary angles word problems

Double facts word problems

Trigonometry word problems

Percentage word problems 

Profit and loss word problems 

Markup and markdown word problems 

Decimal word problems

Word problems on fractions

Word problems on mixed fractrions

One step equation word problems

Linear inequalities word problems

Ratio and proportion word problems

Time and work word problems

Word problems on sets and venn diagrams

Word problems on ages

Pythagorean theorem word problems

Percent of a number word problems

Word problems on constant speed

Word problems on average speed 

Word problems on sum of the angles of a triangle is 180 degree


Profit and loss shortcuts

Percentage shortcuts

Times table shortcuts

Time, speed and distance shortcuts

Ratio and proportion shortcuts

Domain and range of rational functions

Domain and range of rational functions with holes

Graphing rational functions

Graphing rational functions with holes

Converting repeating decimals in to fractions

Decimal representation of rational numbers

Finding square root using long division

L.C.M method to solve time and work problems

Translating the word problems in to algebraic expressions

Remainder when 2 power 256 is divided by 17

Remainder when 17 power 23 is divided by 16

Sum of all three digit numbers divisible by 6

Sum of all three digit numbers divisible by 7

Sum of all three digit numbers divisible by 8

Sum of all three digit numbers formed using 1, 3, 4

Sum of all three four digit numbers formed with non zero digits

Sum of all three four digit numbers formed using 0, 1, 2, 3

Sum of all three four digit numbers formed using 1, 2, 5, 6