Question

n old brainteaser by the Central Asian scholar Biruni (973−1048): If 10 dirhams (a unit of currency) earn 5 dirhams of profit in 2 months, how much profit will 8 dirhams earn in 3 months?

Answer 1

This is a tricky question because you might use the ratio and proportion method. However, there are 3 terms involved. To cut it down to 2, let's lump the profit rate. The solution is as follows:

10:5/2::8:x/3

Solving for x,
20/5 = 24/x
x = 24(5)/20 = 6

Hence, it will earn 6 dirhams of profit.