Each divisor corresponds to a perfect square divisor $s^2$, so the number of such $s$ (i.e., integer side lengths of square grids dividing the area) is equal to the number of positive divisors of $2025$, which is $15$. However, each divisor $d$ of $2025$ corresponds to $s = \sqrtd$, but only those $d$ that are perfect squares yield integer $s$. Since $2025 = 45^2$, the number of square divisors equals the number of perfect square divisors. - Nelissen Grade advocaten