In how many years will a sum of ₹320 amount to ₹405 if interest is compounded at 12.5% per annum?

2 years

 Formula used-

Amount = P$(1 \;+\; \frac{R}{100})^t$

Here, Compound interest = Rs 320 ,  Amount = 405 , R = 12.5%, time = t years 

405 =  320$(1 \;+\; \frac{12.5}{100})^t$

⇒ \(\frac{405}{320}\) =[$(\frac{9}{8})^t $]

⇒ \(\frac{81}{64}\) = [$(\frac{9}{8})^t $]

⇒ t = 2 years

So, time is 2 years.