Question

10 cards are numbered from 1 to 10 and placed in a box. One card is selected at random and is not replaced. Another card is then randomly selected. What is the probability of selecting two prime numbers?

A) 2/15
B) 4/18
C) 7/90
D) 1/10

Answer 1

The primes in the set [1,10] are 1,2,3,5,7 and the number of elements is 10 so

(5/10)(4/9)=20/90=2/9

which for some reason they converted to 4/18 :P