Q.In how many years will ₹5,000 amount to ₹6,050 at 10% per annum, compounded yearly?

The compound-interest formula $A = P\left(1+\dfrac{r}{100n}\right)^{nt}$ relates four quantities — $P$, $r$, $t$ (via $n$) and $A$. Given any three, the fourth can be found: solve for $P$ by dividing $A$ by $\left(1+\dfrac{r}{100n}\right)^{nt}$; solve for the rate or the time by recognising $\dfrac{A}{P}$ as a power of $\left(1+\dfrac{r}{100n}\right)$ — in this chapter's problems, the numbers are chosen so this power can be recognised directly (as a perfect square, cube, etc.) without needing logarithms, which this syllabus introduces only in a later chapter.