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. For how many years was the sum invested?  Choose the correct choice from the following choices.
A) 25 years
B) 40 years
C) 33 years and 4 months
D) Cannot be determined
E) None of these
Solution:   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 = pnr/100, 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 X n X 4)/100   =&gt; n = 100/4= 
<gadget id="calculator">100/4</gadget>
<output>25</output>
25 years.   25 yearsnswer 25 years
<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. For how many years was the sum invested?