You invest $4000 in an account that grows up to $6,520 in 4 years. Find the annual interest rate

Question

Finding the annual interest rate, after 4 years, invest $4000 in an account that grows to $6,520

Answer 1

$6520-$4000=$2520 in interest

interest=principal*rate*time

2520=4000*r*4

divide both sides by 4

630=4000*r

divide both sides by 10

63=400*r

divide both sides by 400

r=63/400

divide both terms by 4

r=(63/4)/(400/4)

r=15.75/100

r=15.75%

also you could have divided both sides by 16,000 to get r=2520/16,000=0.1575=15.75%