In how many years will ₹1000 amount to ₹1331 at 10% per annum, compounded annually?

(1.1)ⁿ = 1331/1000 = 1.331, and $1.331=(1.1{)}^3$, so $n=3$. (Year by year: 1000 → 1100 → 1210 → 1331.) The interest earned is ₹331, which is 33.1% of the sum, but dividing that by the 10% rate to get 3.31 years mixes compound growth with simple-interest reasoning — it works out close only because three years is a short spell.