Probability question

[allthebest]

four different numbers are randomly chosen from the set S= {1,2,3,4,5,6,7,8,9,10}

X is the second largest of the numbers selected

Determine the probability that X is:

a) 2 (=0)

b) 7

c) 9

How do you do b &c

ans for b is 3/14, ans for c is 2/15

[kw0573]

Same logic in b) and c), I'll only do b)
P(X) = favourable outcomes / total outcomes
if X = 7, there can be 3 choices for the largest number. There can be (6 choose 2) choices of the two smaller numbers. The product of these two independent combinations are divided by total outcome: (10 choose 4)
3 * 6C2 / 10C4 = 3 * 15 / 210 = 3/14. 
Similar logic for c)