Arthur D. answered • 04/28/16

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$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%