Principal trebled after 5 years — total simple interest in 10 years: The simple interest on a sum of money is ₹ 600 after 10 years at a constant rate. If the principal is tripled after 5 years (rate unchanged), what will be the total interest by the end of the 10th year?

Introduction / Context:
When principal changes midstream under simple interest, compute interest for each phase separately using the same rate, and add the results. The given 10-year SI on the original setup lets us extract P * r directly.

Given Data / Assumptions:

• SI for 10 years (baseline) = ₹ 600
• After 5 years, principal becomes 3P (rate unchanged)
• We need total SI over 10 years with this change

Concept / Approach:
From I = P * r * t / 100, P * r = 600 * 100 / 10 = 6000. Compute I1 for first 5 years at P, and I2 for next 5 years at 3P, then add.

Step-by-Step Solution:
I1 = (P * r * 5) / 100 = 6000 * 5 / 100 = ₹ 300I2 = (3P * r * 5) / 100 = 3 * 6000 * 5 / 100 = ₹ 900Total interest = 300 + 900 = ₹ 1200

Verification / Alternative check:
Linearity of SI with respect to principal supports summing phase-wise results.

Why Other Options Are Wrong:
₹ 600 and ₹ 900 ignore the tripling in the second phase; ₹ 1500 overshoots due to double counting or compounding.

Common Pitfalls:
Accidentally using compound interest, or assuming the given ₹ 600 is for 5 years instead of 10 years.

Final Answer:
₹ 1200