Final answer:
Monu's gain over a period of 3 years is Rs. 1080.
Step-by-step explanation:
To calculate Monu's gain over a period of 3 years, we need to find the simple interest earned by Monu on the loan he made to Rahul. Monu lent Rs. 3,000 to Rahul at an interest rate of 12% per annum.
The simple interest formula is: Simple Interest = (Principal × Rate × Time) / 100
Using this formula, we can calculate the simple interest earned by Monu over 3 years as follows: Simple Interest = (3000 × 12 × 3) / 100 = Rs. 1080
Therefore, Monu's gain over a period of 3 years is Rs. 1080.