Compound Interest:
Compound interest is the interest calculated on the initial principal as well as the accumulated interest from previous periods. It is a powerful concept that can help individuals and businesses grow their wealth over time.

Formula for Compound Interest:
The formula to calculate compound interest is:
A = P(1 + r/n)^(nt) - P
Where,
A = the final amount (including principal and interest)
P = the principal amount (the initial sum of money)
r = the annual interest rate (expressed as a decimal)
n = the number of times that interest is compounded per year
t = the number of years

Given Information:
In this question, we are given that a sum of money becomes four-fold in 2 years. Let's assume the principal amount as P and the rate of interest as r.

Calculating Compound Interest:
Using the given information, we can set up the equation as follows:
4P = P(1 + r/n)^(nt) - P

Simplifying the equation, we get:
3P = P(1 + r/n)^(nt)

Breaking Down the Equation:
Let's break down the equation to understand it better:
- The left-hand side (3P) represents the interest earned on the principal amount.
- The right-hand side (P(1 + r/n)^(nt)) represents the principal amount plus the interest earned.

Identifying the Variables:
From the equation, we can identify the following variables:
- Principal amount (P)
- Rate of interest (r)
- Number of times interest is compounded per year (n)
- Number of years (t)

Solving for Rate of Interest:
To find the rate of interest (r), we need to know the values of P, n, and t. Once we have those values, we can substitute them into the equation and solve for r.

Additional Considerations:
It's important to note that the given information does not provide the exact values of P, n, and t. Without these values, we cannot calculate the rate of interest accurately.