Explanation:
50% interest annually.
that means he gets 50% interest of the invested capital every year.
and that means he gets 50% of $70 in one year.
70 = 100%
1% = 100%/100 = 70/100 = $0.70
50% = 1%×50 = 0.7 × 50 = $35
he will earn $35 interest in one year.
as you noticed: 50% simply means 1/2 (as 100% stands for the whole).