Given information:
- Amount of money after 1 year: Rs 6200
- Amount of money after 3 years: Rs 7400

Calculating the principal:
To find the principal, we need to subtract the interest earned from the total amount after 1 year.
Principal = Amount after 1 year - Interest earned

Calculating the interest earned:
The interest earned can be calculated by subtracting the principal from the total amount after 3 years.
Interest earned = Amount after 3 years - Principal

Calculating the rate of interest:
The rate of interest can be calculated using the formula:

Rate of interest = (Interest earned / Principal) * 100

Solution:
Let's calculate the principal first.

Principal = Amount after 1 year - Interest earned
Principal = Rs 6200 - Interest earned

Next, let's calculate the interest earned.

Interest earned = Amount after 3 years - Principal
Interest earned = Rs 7400 - Principal

Now, we can substitute the values of Principal and Interest earned in the rate of interest formula.

Rate of interest = (Interest earned / Principal) * 100
Rate of interest = (Rs 7400 - Principal) / Principal * 100

To simplify the calculation, let's assume the principal as 'P'.

Rate of interest = (Rs 7400 - P) / P * 100

Now, let's substitute the value of Principal in the first equation.

P = Rs 6200 - Interest earned
P = Rs 6200 - [(Rs 7400 - P) / P * 100]

Simplifying the equation further:

P = Rs 6200 - (740000/P) + 100P
P^2 - 6200P + 740000 = 0

Solving this quadratic equation using the quadratic formula, we can find the value of Principal.

Once we have the value of Principal, we can substitute it back into the rate of interest formula to find the rate of interest.