Question

Rs 1000 invested at 5% p.a. simple interest. If the interest is added to the principal after every 10 years, the amount will become Rs 2000 after 

(a) 15 years 

(b)   \(6\frac23\) years 

(c) 18 years 

(d) 20 years

Answer 1

(b)   \(6\frac23\) years.

S.I. for 10 years = Rs \(\bigg(1000\times\frac{5}{100}\times10\bigg)\)

= Rs 500

Principal after 10 years becomes 

= Rs (1000 + 500) 

= Rs 1500 

Amount on that principal after t years = Rs 2000 

∴ S.I. on it = Rs (2000 – 1500) = Rs 500

∴ t = \(\bigg(\frac{500\times100}{1500\times5}\bigg)\) years = \(6\frac23\) years

∴ Total time = \(\bigg(10+6\frac{2}3{}\bigg)\)years = \(6\frac23\) years.