Question

You put $400 in a account. The account earns $18 simple interest in 9 months. What is the annual interest rate?

Answer 1

0.06 or 6%

Explanation:

I=Prt, so r= 1/(Pt)

T= 9 months/12 months=0.75

R=$18/($400*0.75)=0.06 or 6%

Note the * is times

Answer 2

Use the formula,
I = Prt
P = Principal amount = 400
I = interest = 18
r = annual rate
t = time in years = 9/12 = 3/4

so,
18 = 400*r*3/4

r = 0.06
so the annual interest rate is 0.06 or 6%.