Round Figure :
A two-digit number will be considered as a round figure, if it has zero at ones place
Examples :
10, 20, 50
A three-digit number will be considered as round figure, if it has zeros at tens place and ones place.
Examples :
100, 200, 600
Method 1 :
Consider the following subtraction :
13 - 7
What number has to be subtracted from 13, so that, it will become a round figure?
If we subtract 3 from 13, it will become 10, which is a round figure.
Now, write 13 as (10 + 3) and -7 as (-3 - 4).
= 13 - 7
= 10 + 3 - 3 - 4
= 10 - 4
= 6
The strategy is to remove a value from the two-digit number such that it will become a round figure and do the rest as explained in the above example.
Method 2 :
Incase, the one-digit number is very close to a round figure 10, we can do the subtraction as follows.
Consider the following subtraction :
28 - 9
Here, 9 is very close to the round figure 10. So, we can write -9 as (-10 + 1).
= 28 - 9
= 28 - 10 + 1
= 18 + 1
= 19
Method 1 :
Consider the following subtraction :
38 - 23
Break the second two-digit number 23 into tens and ones. That is, write -23 as (-20 - 3).
Because, subtracting round figure 20 from 38 is easy. Then, subtract 3 from the result.
= 38 - 23
= 38 - 20 - 3
= 18 - 3
= 15
The strategy is to write the second two-digit number in terms of a round figure and do the rest as explained in the above example.
Method 2 :
Incase, the second-digit number is very close to a round figure, we can do the subtraction as follows.
Consider the following subtraction :
43 - 18
Here, 18 is very close to the round figure 20. So, we can write -18 as (-20 + 2).
= 43 - 18
= 43 - 20 + 2
= 23 + 2
= 25
We can apply the same two methods to subtract a two-digit number from a three-digit number mentally.
In each case, evaluate the expression.
Problem 1 :
43 - 4
Solution :
= 43 - 4
= 40 + 3 - 3 - 1
= 40 - 1
= 39
Problem 2 :
52 - 6
Solution :
= 52 - 6
= 52 - 2 - 4
= 50 - 4
= 46
Problem 3 :
47 - 9
Solution :
= 47 - 9
= 47 - 10 + 1
= 37 + 1
= 38
Problem 4 :
64 - 8
Solution :
= 64 - 8
= 64 - 10 + 2
= 54 + 2
= 56
Problem 5 :
38 - 23
Solution :
= 38 - 23
= 38 - 20 + 3
= 18 + 3
= 21
Problem 6 :
47 - 19
Solution :
= 47 - 19
= 47 - 20 + 1
= 27 + 1
= 28
Problem 7 :
54 - 29
Solution :
= 54 - 29
= 54 - 30 + 1
= 24 + 1
= 25
Problem 8 :
358 - 14
Solution :
= 358 - 14
= 358 - 10 - 4
= 348 - 4
= 344
Problem 9 :
786 - 15
Solution :
= 786 - 15
= 786 - 10 - 5
= 776 - 5
= 771
Problem 10 :
276 - 39
Solution :
= 276 - 39
= 276 - 40 + 1
= 236 + 1
= 237
Problem 11 :
346 - 145
Solution :
= 346 - 145
= (300 - 100) + (46 - 45)
= 200 + 1
= 201
Problem 12 :
456 - 272
Solution :
= 456 - 272
= (400 - 200) + (56 - 72)
= 200 - 16
= 184
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