Answer:
0.2333 = 23.33% probability he passes exactly one test
Explanation:
We have these following probabilities:
3/4 probability that he passes the first test.
If he passes the first test, 4/5 probability that he passes the math test.
If he does not pass the first test, 1/3 probability that he passes the math test.
What is the probability he passes exactly one test
Two scenarios:
Pass first(3/4 probability) and fails second(1/5 probability).
Fails first(1/4 probability) and passes second(1/3 probability). So

0.2333 = 23.33% probability he passes exactly one test