A gardener has 1000 saplings. He plants as many as he can so that the number of rows equals the number of saplings in each row, as sketched. What is the least number of saplings left over?

**Step 1**: factorise the number into primes.
**Step 2**: pair up identical factors.
**Step 3**: from each pair, take ONE factor — the product is the square root.

Example: $\sqrt{324}$. Factorise: $324=2^2\times 3^4=(2\times 3\times 3{)}^2$. So $\sqrt{324}=2\times 3\times 3=18$.

Example: $\sqrt{576}$. Factorise: $576=2^6\times 3^2=(2^3\times 3{)}^2$. So $\sqrt{576}=8\times 3=24$.

If any prime appears an odd number of times, the number isn't a perfect square — and you'd need to multiply (or divide) by that prime to make it one.