Given sum = 10000

Rate of interest = 5%

Interest after one year = (10000*5*1)/100 = 500

So amount after one year = 10000 + 500 = 10500

Again amount after two years = 10000 + 2* 500 = 10000 + 1000 = 11000

Again amount after thrr years = 10000 + 3* 500 = 10000 + 1500 = 11500

and so on.

Now it forms an AP and the seroes is: 10000, 10500, 11000, 11500,........ 

Now first term a = 10000 and common difference d = 500

Now 15th term = a+ (15-1)*d

                      = 10000 + 14*500

                     = 10000 + 7000

                     = 17000

So amount in 15th year = 17000 rupees

 Now again amount after 20 years = 10000 + SI for 20 years

                                                  = 10000 + (10000*5*20)/100

                                                  = 10000 + 100*100

                                                  = 10000 + 10000

                                                  = 20000 rupees