A man borrowed a sum of Rs.12000 from bank at simple interest. After 3 years he paid Rs.4000 and at the end of 5thyears he paid 3000 and at the end of 9thyear he paid Rs.7520 and clears his dues. Find the rate of interest per annum.

3.5%

Total interest paid = (₨.4000 + ₨.3000 + ₨.7520) - (₨.12000) = ₨.2520

Interest = interest on ₨.12000 for 3 years + interest on remaining ₨.8000 for 2 years + interest on remaining ₨.5000 for 4 years 

2520 = \(\frac{12000 x 3 xR}{100}\) + \(\frac{8000 x 2 xR}{100}\)+ \(\frac{5000 x 4 xR}{100}\)

R = \(\frac{2520 x 100}{72000}\)

    = 3.5%