The probability that they both get 0 heads is 1/8*1/16, or 1/128.
The probability that they both get 1 head is 3/8*1/4, or 3/32.
THe probability that they both get 2 heads is 3/8*6/16, or 18/128
THe probability that they both get 3 heads is 1/8*1/4, or 1/32.
Add up all those fractions: (im too lazy) to get your answer.
New Problem: AIME15 wants his choo-choo train to run for at least 15 days. However, it starts to break down after the 2nd day. He knows it will take 5 days for the train to completely break down. It takes him 1 hour to earn 5 dollars, and he needs 100 dollars to fix his choo-choo train. THen he has to run to the hardware store (5mph) which is 20 miles away, and buy parts (takes 15 minutes), run back (3 mph because he's weighed down) and fix his train(2 days). Can he make it in time?