A sum borrowed under simple interest doubles itself in 10 years. When will it…

Correct Answer: 30 years

Let the principal be P.
For simple interest:
SI = (P × R × T) / 100

Since the sum doubles in 10 years:
P + (P × R × 10) / 100 = 2P

⇒ (P × R × 10) / 100 = P
⇒ R × 10 / 100 = 1
⇒ R = 10% per annum

To become fourfold:
P + (P × R × T) / 100 = 4P

⇒ (P × R × T) / 100 = 3P
⇒ R × T = 300

With R = 10,
10 × T = 300
⇒ T = 30 years

Therefore, the sum will become four times itself in 30 years.