Simple Interest — A sum becomes double in 10 years. Find the annual rate of simple interest.

Introduction / Context:
Doubling under simple interest implies the interest earned equals the principal over the given time.

Given Data / Assumptions:

• A = 2P in t = 10 years.
• Simple interest applies.

Concept / Approach:
If A = 2P then SI = P. With SI = P * r * t, solve for r.

Step-by-Step Solution:
P = P * r * 10 ⇒ r = 1 / 10 = 0.10 = 10%.

Verification / Alternative check:
At 10% for 10 years, SI = P; amount = 2P.

Why Other Options Are Wrong:
1% and 5% are too small; 20% is too large.

Common Pitfalls:
Applying compound interest ideas; stay with SI linearity.

Final Answer:
10%