The return on debt can be calculated using the formula:

In this case, the face value of the debenture is Rs. 100 and it is issued for Rs. 120. The coupon rate is 12%, which means the annual interest is calculated as 12% of the face value.
The annual interest is
.
The cost of debt is the amount at which the debenture is issued, which is Rs. 120.
Substituting the values into the formula:

Therefore, the return on debt is 10%.