Combination Problems With Solutions :
Here we are going to see some practice questions base d on the concept combination.
Question 1 :
A box contains two white balls, three black balls and four red balls. In how many ways can three balls be drawn from the box, if at least one black ball is to be included in the draw?
Number of white balls = 2
Number of black balls = 3
Number of red balls = 4
Number of non black balls = 2 + 4 = 6
Number of ways
= (3C1 ⋅ 6C2) + (3C2 ⋅ 6C1) + (3C3 ⋅ 6C0)
= (3 ⋅ 15) + (3 ⋅ 6) + (1 ⋅ 1)
= 45 + 18 + 1
Question 2 :
Find the number of strings of 4 letters that can be formed with the letters of the word EXAMINATION?
There are 11 letters not all different.
They are AA, II, NN, E, X, M, T, O.
The following combinations are possible:
Case 1 :
Number of ways selecting 2 alike, 2 alike
= 3C2 = 3 ways
Case 2 :
Number of ways selecting 2 alike,2 different
= 3C1 ⋅ 7C2 ==> 3 x 21 ==> 63 ways.
Case 3 :
Number of ways selecting all 4 different = 8C4
= 70 ways.
Total number of combinations = 3 + 63 + 70 = 136 ways.
Total number of permutations (1) to (3)
= 3 ⋅ (4!/2!2!) + 63 ⋅ (4!/2!) + 70 ⋅ 4!
= 18 + 756 + 1680
Question 3 :
How many triangles can be formed by joining 15 points on the plane, in which no line joining any three points?
From the given question, we come to know that any three points are not collinear.
By selecting any three points out of 15 points, we draw a triangle.
Number of ways to draw a triangle = 15C3
= (15 ⋅ 14 ⋅ 13) / (3 ⋅ 2 ⋅ 1)
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