Please help i have no idea what this is!!

Two airplanes start at the same place and travel in opposite directions, one at 395 miles per hour and the other at 422 miles per hour. how many hours will it take for the planes to be 2451 miles apart?

According to the general equation for conditional probability, if p(ab) = 4/5 and p(b)= 5/6, what is p(a|b)? a. 8/9 b. 35/36 c. 24/25 d. 15/16

30 points if you answer all 3 questions with work!