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A sum of money invested for a certain number of years at 8% p.a. simple interest grows to Rs.180. The same sum of money invested for the same number of years at 4% p.a. simple interest grows to Rs.120 only. For how many years was the sum invested?

Correct Answer: 25years

Explanation:


From the information provided we know that,


Principal + 8% p.a. interest on principal for n years = 180 …….. (1)


Principal + 4% p.a. interest on principal for n years = 120 ……… (2)


Subtracting equation (2) from equation (1), we get


4% p.a. interest on principal for n years = Rs.60.


Now, we can substitute this value in equation (2),


i.e Principal + 60 = 120


= Principal = Rs.60.


We know that SI = , where p is the principal, n the number of years and r the rate percent of  interest.


In equation (2), p = Rs.60, r = 4% p.a. and the simple interest = Rs.60.


Therefore, 60 =(60*n*4)/100


=> n = 100/4 = 25 years.


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