A sum of money triples itself in 12 years when the principal and compound interest are added. In how many years will it become nine times the sum?
24 years
Under compound interest, A = P(1 + r/100)n, so the growth factor multiplies over equal periods.
Given P(1 + r/100)12 = 3P, so (1 + r/100)12 = 3.
Nine times the sum: (1 + r/100)n = 9 = 3²
= [(1 + r/100)12]²
= (1 + r/100)24
n = 24
Check: the sum becomes 3 times in the first 12 years and 3 × 3 = 9 times after another 12 years, that is 24 years in all.
Hence, 24 years.
Important Questions from Compound Interest
A person borrowed Rs. 10000 on compound interest at the rate of 40 percent per annum. If the interest is compounded half yearly, then what will be the amount to be paid after 1.5 years?
The difference between the compound interest (compounding annually) and the simple interest on a sum of money at the rate of 40 per cent per annum for 2 years is Rs. 2400. What is the amount?
In how many years will a sum of Rs.1875 amount to Rs.2187 at 8 percent p.a. compound interest?
A sum of money has increased by 45% in 9 years at simple interest. What will be the compound interest of Rs. 12,000 after 3 years at the same rate?
At a certain rate of compound interest a certain sum amounts to Rs. 64800 in 4 years and Rs. 93312 in 6 years. What is the compound interest earned in fifth year?
