A sum of money becomes four times its principal In 18 years after earning simple interest for this duration. At what rate of interest was it invested?

Explanation:

When a sum of money becomes four times its principal,

it means that the Interest earned = 3 times the principal.

Let P be the principal, Therefore, the simple interest earned in the 18 years will be = 3P.

Now, 3P = P×18×r\100 or300=18r

Or the rate at which the money was invested, r = 300\18=100\6=16.67%