At what annual simple-interest rate will a sum of money double in 8 years?

Introduction / Context:
Under simple interest, amount A = P * (1 + r * t). Doubling means A = 2P, so 1 + r * t = 2 → r * t = 1.

Given Data / Assumptions:

• t = 8 years.
• Simple interest model.

Concept / Approach:
r = 1 / t (in decimal).

Step-by-Step Solution:
r = 1 / 8 = 0.125 = 12.5% per annum.

Verification / Alternative check:
Check: A = P (1 + 0.125 * 8) = P * 2 = 2P.

Why Other Options Are Wrong:
12% gives 1.96P; 13% gives 2.04P; 15% gives 2.20P in 8 years.

Common Pitfalls:
Applying compound-interest logic; here the model is simple interest.

Final Answer:
12.5 %