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? Choose the correct choice from the following answers:
A) 25
B) 37
C) 36
D) 29
E) 21
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   =&gt; n = 100/4= 
<gadget id="calculator">100/4</gadget>
<output>25</output>
25 years.   25nswer: 25) 25years
<result>A</result>
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?