Easy question.
(1)
Let's assume x miles from A to B.
Then needs x/10 hours from point A to point B,
needs x/B hours from point B to point A.
So spent (x/10 + x/B) hours from point A to B.
Drove 2*x miles. (A-> B->A, round trip)
So the AVG speed is 2*x / (x/10 + x/B) = 2 / (0.1 + 1/B)
Use a calculator, you will get your answer.
(2) If the AVG speed is 15mph, then 15 = 2/(0.1 + 1/B),
so the speed from point B to point A = 30mph.
))