13. If the interest on Rs. 1200 be more than the interest on Rs. 1000 by Rs. 50 in 3 years, the rate per cent is:

14. The simple interest on a sum of money is of the principal and the number of years is equal to the rate per cent per annum. The rate percent per annum is:

15. In how many years will a sum of money double itself at 12% per annum: