Let x be the number of students in the first class and y the number of students in the second class.
We find x and y.
x + y = 50 and
x - y = 10
Adding these equations, we find that
2x +y -y = 50+10,
2x = 60 and so
x = 30
Then y = 30 - 10 = 20
So, taking into account both classes,
0.7*30 + 0.8*20 = 21 + 16 = 37 students passed.
Therefore 37/50 = 0.74 or 74%,
so the correct answer is (E) 74%.