Simple Interest [साधारण ब्याज-06

Question: A certain sum is invested for certain time. It amounts to Rs. 80 at 5% per annum. But when invested at 2% per annum, it amounts to Rs. 40. Find the time?[Ans.(a)100 yrs]

(a) 100 years (b) 120 years (c) 90 years (d) 110 years (e) None of these

Solution: 

Detailed Explanation: Let a certain sum “P”, invested for a certain time “t” years, becomes Rs. 80 at 5% pan and Rs. 40 at 2% pan.

Therefore, (100+5t) % of P = 80 and (100+2t) % of P = 40

So, [(100+5t) % of P]/[ (100+2t) % of P] = 80/40 or (100+5t)/(100+2t) = 2/1 or,

t = 100 years