Business maths help please!!

[Angelz369]

Brad purchases a bed for $1100.  he pays a deposit of $300 and quarterly repayments of $230 for one year.  The effective interest rate that Brad will be paying is approximately:
Answer = 24%
The working on the solutions says:
Brad pays a total of 300 + 4x230 = $1220
Total interst = 1220 - 1100=$120 interest
Flate rate of interest = (120x100/800) = 15%

I get everything and how to find effective rate but I don't get the flate rate of interest calculation!!
Why is it only dividing by $800?? aren't you meant to also times it by the number of years so since it's quarterly repayments don't you multiply 800 with 1/3?

[MJRomeo81]

Flat rate of interest is:

(100 * I) / (P * T);

* where I interest is 1220 - 1100 = $ 120

* where P is the principle (we never include the deposit as part of the principle in hire purchase. So 1100-300 = 800)

* where t is how many years? Note, this is different from "n" (n meaning how many payments?). So T is 1 for one year
Thus the equation becomes,

(100* 120) / (800 * 1) = 15 % flat rate

[Angelz369]

Flat rate of interest is:

(100 * I) / (P * T);

* where I interest is 1220 - 1100 = $ 120

* where P is the principle (we never include the deposit as part of the principle in hire purchase. So 1100-300 = 800)

* where t is how many years? Note, this is different from "n" (n meaning how many payments?). So T is 1 for one year
Thus the equation becomes,

(100* 120) / (800 * 1) = 15 % flat rate

oohh ok thanks!!