A certain amount will become six times in 20 years. How long does it take for the same amount to become 5 times? Assume the same rate of simple interest in each case.

16 years

let principle = Rs100

100 becomes 600 in 20 years

So Interest = 600-100=Rs 500

Now 100  becomes 500 (5 times)

∴ Interest = Rs 400

Interest becomes \(\frac{4}{5}\) times

Then time also becomes \(\frac{4}{5}\) times 

∴ \(\frac{20×4}{5}\) = 16 years