To calculate the total interest payable, we use the formula
I=Prt
where:
I = simple interest
P = $2,400
r = 0.08
t = 18/12
Then I = 2,400 x 0.08 x 1.5 = 288
Therefore the total amount he receives at loan drawdown is $2,400 − $288 =$2,112.
Now to calculate the effective interest rate we use the formula
A=P(1+rext).
where
A = $2,400
P = $2,112
t = 1.5
re = effective interest rate
so we have: 2,400 = 2,112(1 + 1.5xre)
finally we solve for re and we obtain
re = 9.1