A certain sum doubles in 3 years under simple interest. In how many years will the sum become 5 times itself?

A certain sum becomes 3 times itself in 6 years at simple interest. In how many years will it become 9 times itself ?

A sum of money becomes 3 times in 5 years at simple interest. In how many years will the same sum become 6 times at the same rate of simple interest?

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?