Carson drives to school the same way each day, and there are two independent traffic lights on his trip to school. He knows that there is a 30% chance that he will have to stop at the first light and an 80% chance that he will have to stop at the second light. What is the probability that he will NOT have to stop at either light?

Question

A) 14%
B) 24%
C) 50%
D) 80%

Answer 1

I think the answer is C) 50%

Answer 2

How did you come up with 50%?

Answer 3

I am not exactly sure but... Since there is a 30% chance that he will have to stop at the first light, there is a 70% chance that he won't. And since there is an 80% chance he will have to stop at the second light, there is a 20% chance he won't.

So I subtracted -> 70% - 20% = 50%

If there is a 70% chance he won't have to stop at the first light, and a 20% chance he won't have to stop at the second light, then there is a 50% chance he won't have stop at either light.

(By the way, have you learned how to solve these types of probability problems already?)