To determine how many lawns Gio needs to mow to reach his savings goal of $900, we first need to find out how much more money he needs.
He already has $250, and his goal is $900. Therefore, the amount he still needs is:
\[ 900 - 250 = 650 \]
Gio earns $25 for each lawn he mows. To find out how many lawns he needs to mow to make an additional $650, we divide the additional amount needed by the amount he earns per lawn:
\[ \frac{650}{25} = 26 \]
Thus, Gio needs to mow 26 lawns to reach his savings goal of $900.