If sum become n time in t years then rate of interest

At r% per annum a sum become 2 times after 20 years. The rate of interest per annum is :

A certain sum becomes 16 times in 4 years at compound interest, compounded annually. What is the rate of interest?

If a certain sum becomes 2 times in 7 years at compound interest, then in how many years, it will become 8 times?

If certain sum becomes 3 times itself in 4 years at compound interest, in how many years will the sum become 81 times itself at the same rate of interest?