Shortcut Trick
On simple interest, a certain sum becomes Rs. ₹49,600 in 3 years and ₹56,000 in 5 years.
The total interest in 2 years = 56000 - 49600 = Rs. 6400
The total interest in 1 year = 6400/2 = Rs. 3200
The total interest in 5 years = 3200 × 5 = Rs. 16000
So, the total amount = 56000 - 16000 = Rs. 40000
The rate of percentage = (3200/40000) × 100 = 8%
The new rate of percentage = 8 + 2 = 10%
So, time required 100%/10% = 10 years
∴ The sum have become doubled in 10 years
Alternate Method
Given:
On simple interest, a certain sum becomes Rs. ₹49,600 in 3 years and ₹56,000 in 5 years.
Formula used:
The simple interest = (p × r × t)/100
Where, p = Total amount
r = Rate of interest
t = Time (years)
Calculation:
Let, the amount = P and Rate of interest = R
So, (P × R × 3)/100 = (49600 - P)
⇒ (P × R)/100 = (49600 - P)/3 .....(1)
Also, (P × R × 5)/100 = (56000 - P)
⇒ (P × R)/100 = (56000 - P)/5 .....(2)
Then, (49600 - P)/3 = (56000 - P)/5
⇒ 168000 - 3P = 248000 - 5P
⇒ 5P - 3P = 248000 - 168000
⇒ 2P = 80000
⇒ P = 80000/2 = 40000
∴ The amount is 40000
Putting P = 40000 in the equation (1),
⇒ (40000 × R)/100 = (49600 - 40000)/3
⇒ 400 × R = 9600/3
⇒ R = 9600/(3 × 400) = 8
∴ The rate of interest = 8%
The required interest = (2 × 40000) - 40000 = 40000
The rate of interest = 8 + 2 = 10%
Let, the time required = T
So, (40000 × 10 × T)/100 = 40000
⇒ 400 × 10 × T = 40000
⇒ T = 40000/(400 × 10) = 10
∴ The sum have become doubled in 10 years