What is the least number to be added to 1500 to make it a perfect square

Let's find the squares of numbers close to $1500$:
$$38^2=1444$$

$$39^2=1521$$

Since $1444<1500<1521$, the next perfect square is $1521$.
The least number to be added to $1500$ to make it a perfect square is:

$$1521-1500=21$$