percentage problem

Answer: 20%

Solution:
Let R be the interest rate
Let $P be the principal and so the amount after 10 years is $3P

amount = principal + interest
amount = principal + principal x interest rate x time
3P = P + P x R x 10
2P = P x R x 10
2 = R x 10
R = 0.2

So the interest rate is 0.2 or 20%