Further Mathematics - Business Related Math Question Helps

[Jacques.B]

If anyone has completed business related math in the Cambridge Essentials Further Mathematics 3 and 4, i need help with one of the questions in exercise 21C, question (1) a)

if you do not have the book, the question is:

1) Calculate the flat interest rate per annum on each of the following loans:
   A) $2400, to be repaid in 30 monthly instalments $90

*if a deposit amount was given then it would be much easier to find the answer.

Thanks in advance

[lukaal340]

interest is (30x90)-2400 (total amount payed less actual loan)
so interest is $300
time is 30/12 (30 months divided by 12 to convert it to a yearly figure)
principle is $2400
and rate is what your finding, so transpose I=(PRT)/100 for r (or use solve function on calculator)
 answer is 5% p.a
if ive made any mistakes someone correct me