An amount becomes double in 7 years on simple interest. The amount would be four times in ______ years on the same rate of simple interest.

Given:
Amount doubles in 7 years (Simple Interest)

Concept:

In Simple Interest, interest is directly proportional to time.

If amount becomes double:
Interest = Principal in 7 years

So, rate per year gives $\frac{1P}{7}$ per year

To make amount four times:

Final amount = 4P
So, interest = 4P − P = 3P

Time required:

If 1P interest → 7 years
Then 3P interest → $3 \times 7 = 21$ years

Final Answer: 21 years