Question

Q40) An amount doubles in 7 years when invested under compound interest at a certain rate. In how many years will the amount become 8 times at the same rate?

• A. (a) 9 Years
  
• B. (b) 14 Years
  
• C. (c) 21 Years
• D. (d) 28 Years

Answer 1

The correct option is C (c) 21 Years

Ans:40)(c) Sol: 2P=P(1+r/100)7⇒(1+r/100)7=2;
8P= P(1+r/100)x
⇒8=(1+r/100)x ⇒23=(1+r/100)x
⇒((1+r/100)7)3 =(1+r/100)x
⇒ x=7×3=21 years.
Shortcut:-If years double, amount will be a²
If years triple, amount will be a³.
If number of years is multiplied by x, then amount will be a^x