Question

Jim can't remember the last digit of his best friend's phone number. He plans to just dial the number and guess the last digit. What's the probability that he'll dial the correct number in the first 2 tries?

a) 1/10
b) 1/45
c) 17/10
d) 1/5
e) 2/5

Answer 1

D.

Explanation:

1) the required probability can be calculated as sum of two probabilities:

P=p1+p2, where p1 - when the 1st attempt is correct, p2 - when the 2d attempt is correct;

2) p1=1/10; p2=1/9, then the required probability:

P=1/10 + 1/9 ≈ 1/5.