If a sum of money doubles itself on simple interest in 7 years, then in how many years will it become 4 times of itself ?

21

Let Principal = P , Amount = 2P , Time = 7 years and rate = R%

Amount = Principal + SI

SI = A - P

= 2P  - P = P

SI = \(\frac{P × R × T}{100}\)

P = \(\frac{P × R × 7}{100}\)

R = \(\frac{100}{7}\)%

When amount got 4 times ,

A = 4P , Principal = P

SI = 3P

Now,

SI = \(\frac{P × R × T}{100}\)

3P = \(\frac{P × 100 × T}{100 × 7}\)

T = 21 years

The correct answer is option (4) : 21