A sum doubles in seven years at simple interest. In how many years will the sum become five times the original sum?

28

Given :-

Sum doubles in seven years at simple interest.

Let sum = P , Amount = 2P

So , SI= 2P - P = P

P = \(\frac{P ×Rate × 7}{100}\)

Rate = \(\frac{100}{7}\)%

Now ,

Sum become 5 times of itself . SO , SI = 5P - P = 4P

4P = \(\frac{P × 100 × Time }{100 × 7}\)

Time = 28 years