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?

Explanation

To solve the problem step by step, let's break it down:

### Step 1: Understand the Problem
We know that a certain sum (let's call it P) becomes 3 times itself in 4 years at compound interest. This means:
\[ A = 3P \]
where A is the amount after 4 years.

### Step 2: Use the Compound Interest Formula
...