Let the running time for movie b = x minutes
Running time for movie a = (x + 39) minutes
3x - (39 + x ) = 47 minutes
3x - x = 47 + 39 minutes
2x = 86 minutes, x = 43 minutes
So movie b runs 43 minutes
Movie a runs 43 + 39 = 82 minutes
He will need 2/3 of a cup.
You can do this by setting up a proportion, so
(2/3)/(3/4) = x/1.
Then you would get 2/3 = 3/4x
Divide both sides by 3/4
X = 8/9
15 - 8
8 + 2 is 10, so there are 5 leftover. 2 + 5 is 7, so 8 + 7 is 15.