A sum of money doubles itself in 10 years at simple interest. What is the annual interest rate?

Question: A sum of money doubles itself in 10 years at simple interest. What is the annual interest rate?

Solution:
Let,
Principal amount = P
Sum of amount = 2P
∴ Interest, I = 2P - P = P
Time, n = 10 years
Rate of interest = r

We know,
I = Pnr
∴ r = I/Pn
= P/(P × 10)
= 1/10 × 100%
= 10%