Simple Interest — Borrow at 9%, add own money, lend at 12%: A borrows ₹ 4800 at 9% p.a. SI for 3 years. He adds some of his own money and lends the total at 12% p.a. SI for the same 3 years. If his net gain is ₹ 720, how much did he add?

Introduction / Context:
Net profit in a borrow-and-lend setup equals interest earned on the lent total minus interest paid on the borrowed amount. With equal terms and simple interest, it becomes a straightforward linear equation in the added amount.

Given Data / Assumptions:

• Borrowed principal = ₹ 4800 at 9% for 3 years
• Let added amount = m; lent principal = 4800 + m at 12% for 3 years
• Net gain over 3 years = ₹ 720

Concept / Approach:
Profit = (4800 + m) * 0.12 * 3 − 4800 * 0.09 * 3. Set equal to 720 and solve for m.

Step-by-Step Solution:

Verification / Alternative check:

Why Other Options Are Wrong:

• 500, 640, 740, 900 do not balance the profit equation to 720.

Common Pitfalls:

• Mixing rates or durations; both are 3 years so factors are comparable.

Final Answer:
₹ 800.