Shortcut Tricks on simple interest
1. If a sum of money becomes n times in T yr at simple interest, then formula for calculating rate of interest will be given as R = {100 (n- 1) / T} %
2. If a sum of money at a certain rate of interest becomes n times in T yr and m times in t yr, then formula for t will be given as t = [( m - 1)/(n - 1)] × T
3.If a sum of money in a certain time becomes n times at R rate of interest and m times at r rate of interest, then formula for r will be given as
r = [(m-1)/(n-1)] × R
Questions
Q. A sum of money becomes four times in 20 yr at SI. Find the rate of interest.
Solution
Here T = 20yr, n = 4
Therefore R = {100 ( n - 1) /T} %
= { 100 ( 4 - 1) / 20}%
= 15 %
Q. A sum becomes two times in 5 yr at a certain rate of interest. Find the time in which the same amount will be 8 times at the same rate of interest.
Solution
Here n = 2, m = 8, T = 5, t =?
t = [( m - 1)/(n - 1)] × T
= [(8 - 1)/(2 - 1)] × 5
= 35
Q. In a certain time,a sum becomes 3 times at the rate of 5% per annum. At what rate of interest, the same sum becomes 6 times in same duration?
Solution
Here n = 3, m = 6, R = 5%, r =?
r = [(m - 1)/(n - 1)] × R
= [(6 - 1)/( 3 - 1)] × 5
= 25 /2 = 12.5%
Q. What would be the simple interest accrued in 4 yr on a principle of X 18440 at the rate 15% per annum? [IBPS CLERK 2011]
Solution
Given P = 18440, R = 15%, T = 4
Therefore,
Simple Interest (S. I.) = P × R × T / 100
= 18440 × 15 × 4 / 100
= 11064
Q. A sum becomes its double in 10 yr. Find the annual rate of simple interest.
Solution
Here n = 2, T = 10
Therefore
R = 100 (n - 1) / T
= 100 (2 - 1) / 10
= 10 %
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