Gio mows lawns for 25 each. He already has 250 in his bank account, but is trying to save $900. How many lawns will he need to mow to reach his goal?

Gio wants to save $900 and already has $250, so he needs to save an additional $900 - $250 = $<<900-250=650>>650.

Since he gets $25 for each lawn mowed, Gio will need to mow $650 / $25 per lawn = <<650/25=26>>26 lawns. Answer: \boxed{26}.