Gopal borrowed some money at 12% simple interest. If he had to pay back Rs. 1280 after 5 years, in order to clear off the loan. How much did he borrow?

Let the principle amount be Rs. P

Interest rate = 12%

Total amount he paid after 5 years = Rs. 1280

ATQ, 

$P + I = \frac{\mathrm{PTR}}{100} + P1280 = P\frac{\mathrm{TR}}{100} + 11280 = P\frac{5 x 12}{100} + 1\frac{16P}{10} = 1280P = 80 x 10 = 800$

Hence, the amount he borrowed = P = Rs. 800.