In a certain time, a sum becomes 4 times of itself on simple interest at the rate of 10% per annum. What is the rate of interest if the same sum becomes 7 times of itself in the same duration?

Given:
The principal(P) amount becomes 4 times at simple interest(SI) in 10 years

Formula used:

SI = (P × R × T)/100

R = rate of interest

T = time period

Calculation:

According to the question:

4P = P + (P × R × T)/100

⇒ 4P = P[1 + {(T× 10)/100}]

⇒ 40 = 10 + T

⇒ T = 30

∴ Time is 30
Now for 7 times of sum with 30 Time ,
7P = P + (P × R × T)/100
7P= P[1 + {(R× 30)/100}]
7=1 + {(R× 3)/10}
7-1=3R/10
6=3R/10
R=60/3
R=20%
By Samraj Rajapalayam