Simple Interest – Time to quadruple at a given rate: At a simple interest rate of 10% per annum, in how many years will a sum of money become four times its original value?

Introduction / Context:
Find the time needed for a target multiple under simple interest.

Given Data / Assumptions:

• Target: 4P → 1 + r*t = 4
• r = 10% per annum

Concept / Approach:
Use 1 + r*t = multiple. Solve t = (multiple - 1) / r.

Step-by-Step Solution:
1 + 0.10 t = 4 → 0.10 t = 3 → t = 30 years

Verification / Alternative check:
At 10%, 30 years give 300% interest, so amount = 400% of principal = 4P.

Why Other Options Are Wrong:
25, 15, or 40 years produce 350%, 150%, and 400% interest differently; only 30 years fits 3P interest.

Common Pitfalls:
Confusing 4x with 400% interest vs. 300% interest over P.

Final Answer:
30 yr