Multiple choice

A certain sum lent out at simple interest doubles itself in $20$ years. Number of years in which this sum trebles itself at the same rate of interest is:

  1. $\;30$
  2. $\;40$
  3. $\;25$
  4. $\;20$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Simple interest: I = P*r*t. Doubles in 20 years means I = P, so P = P*r*20, r = 1/20 = 5%. To treble, I = 2P. 2P = P*0.05*t. t = 2/0.05 = 40 years.

AI explanation

When a sum doubles itself at simple interest, the interest earned equals the principal, meaning Rs. 100 generates Rs. 100 in 20 years for an interest rate of 5 percent. To treble itself, the interest must be twice the principal, requiring Rs. 100 to generate Rs. 200. At 5 percent, earning Rs. 200 takes 40 years.

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