To do this, you would create an equation for the problem.
[tex] \frac{3,600}{12,000} [/tex] = [tex] \frac{x}{100} [/tex]
Next, you would cross multiply.
12,000x = 360,000
The last step would be to isolate the x. To do this, you would divide both sides by 12,000.
x = 30%
The interest rate would be 30%.
I hope this helps!