The simple interest on Rs. 10 for a period of 4 months is calculated at a rate of 3 paise per rupee per month. When this monthly rate is applied, the interest is how many rupees in total?

Introduction / Context:
This is a straightforward simple interest calculation involving a small principal and a rate given in paise per rupee per month. The question checks whether the student can correctly convert paise to rupees, interpret the rate per month, and apply it for the given number of months to find the total interest in rupees.

Given Data / Assumptions:

• Principal P = Rs. 10.
• Time T = 4 months.
• Rate is 3 paise per rupee per month.
• 100 paise = 1 rupee.
• We use simple interest, so interest each month is constant on the principal.

Concept / Approach:
The rate of 3 paise per rupee per month means that for every rupee of principal, the interest is Rs. 0.03 each month. Therefore, the monthly rate in percentage is 3%. Since the principal is Rs. 10 and the time is 4 months, we can treat the period directly in months and multiply the monthly interest accordingly.

Step-by-Step Solution:
Convert 3 paise to rupees: 3 paise = 3 / 100 = Rs. 0.03. Therefore, monthly rate per rupee is Rs. 0.03, which is 3% of 1 rupee. Interest per month on Rs. 10 = 10 * 3% = 10 * 0.03 = Rs. 0.30. Time is 4 months, so total interest = 4 * 0.30. Total simple interest = Rs. 1.20.
Verification / Alternative check:
We can also apply the simple interest formula in month units: SI = (P * R * T) / 100 with P = 10, R = 3% per month, T = 4 months. So SI = (10 * 3 * 4) / 100 = 120 / 100 = Rs. 1.20. This confirms that the interest is Rs. 1.20.

Why Other Options Are Wrong:
Rs. 0.03 is only the interest on Rs. 1 for one month, not on Rs. 10 for 4 months. Rs. 0.12 and Rs. 0.30 correspond to partial calculations, such as using only one month or incorrect multiplication. Rs. 3.00 greatly overestimates the interest and would require either a much longer time or a much higher rate.

Common Pitfalls:
A common mistake is to forget the conversion between paise and rupees, leading to using 3 rupees instead of Rs. 0.03. Others may apply the rate for only one month or misinterpret it as an annual rate instead of a monthly rate. Careful interpretation of the units avoids these errors.

Final Answer:
The total simple interest earned is Rs. 1.20.