Simple Interest – Rate percent when a sum trebles: A sum of money placed at simple interest becomes three times the principal in 15 years. Find the annual rate of interest (percent per annum).

Introduction / Context:
Trebles means the amount equals 3P, so the interest earned is 2P. With SI, SI = P * r * t; solve for r using t = 15 years.

Given Data / Assumptions:

• A = 3P
• t = 15 years

Concept / Approach:
Since SI = A - P = 2P, we have P * r * 15 = 2P ⇒ r = 2 / 15 per year = 0.1333... = 13 1/3%.

Step-by-Step Solution:

Verification / Alternative check:

Why Other Options Are Wrong:

• 16 2/3% / 12 2/3% / 20% / 10%: do not equal 2 / 15 as a percent.

Common Pitfalls:

• Confusing treble (3P) with double (2P).

Final Answer:
13 1/3% per annum.