Question

Kevin has an equal number of dimes, nickel and quarters in his piggy bank. He randomly picks a coin, replaces it, and picks another coin. What is the probability that the sum of the two coins is at least 30cents?

Answer 1

Say he has 2 dimes (D), 2 nickels (N), and 2 quarters (Q). There are 3 * 3 = 9 choices for what he can pull out. How many will give at least 30 cents?

Q and N, N and Q, Q and D, D and Q

That's 4 out of 9. So, the probability is 4/9.