Meera spends exactly ₹100 on pens costing ₹7 each and notebooks costing ₹11 each, buying at least one of each. How many pens does she buy?

We need whole numbers $p$ and $n$ with $7p+11n=100$. Work through the notebooks: $n=1$ leaves 89, $n=2$ leaves 78, $n=3$ leaves 67 — none divisible by 7 — but $n=4$ leaves $100-44=56=7\times 8$. So $p=8$ and $n=4$, and no other combination works. The trap is 4, which is the number of notebooks rather than the number of pens.