"The difference between the CI and the SI on a sum of money lent for 2 years at 20% interest per annum is ₹ 80. The sum is:"

₹2,000

The formula used here is,

Difference in the simple interest and compound interest on a certain sum at R% per annum in 2 years = (CI - SI) = $\frac{PR^2}{100^2}$

⇒ Difference in the simple interest and compound interest on a certain sum at 20% per annum in 2 years = Rs. 80

⇒ 80 = $\frac{P20^2}{100^2}$

⇒  80  = P x \(\frac{1}{25}\)

⇒   P   = 2000