Question

the difference between the compound interest and the simple interest accrued on and amount of rupees 18000 in 2 years was rupees 405. what was the rate of interest

Answer 1

Let x be the rate of interest. 
Simple interest for 2 years = 18000*x/100*2 = 360x 
Compund Interest for the first year = 18000*x/100 = 180x 
Compund interest for the second year = (18000 + 180x)*x/100 = 1.8x^2 + 180x 
Total Compund interest = 180x + (1.8x^2 + 180x) = 1.8x^2 + 360x 

Difference between simple and compund interest = (1.8x^2 +360x) - 360x = 1.8x^2 = 405 
x^2 = 225 
x = 15