Farmer purchased a tractor for Rs. 12,000. After paying Rs. 6000, he will left with Rs. 6000. 1st interest paid = Rs. $\frac{6000 \times 12}{100}$ = Rs. 720 2nd interest paid = Rs. $\frac{5500 \times 12}{100}$ = Rs . 660 ................... ........................... .......................... 12th interest paid = Rs. $\frac{500 \times 12}{100}$ = Rs. 60 Total amount of interest = Rs. [720 + 660 + ..... + 60] = Rs. $\left[ \frac{12}{2}(720+60) \right]$ Since 720, 660, ...., 60 is an A.P. with n = 12, a = 720, l = 60 = Rs. (6 × 780) = Rs. 4680 ∴ The cost of tractor = Rs. (12000 + 4680) = Rs. 16680.