Answer:
25 sheets of plywood will be needed
Step-by-step explanation:
Each sheet of plywood measures .
We have to cover the surface of the house, minus the surface of the hole of the stairs.
To know how many sheets will be needed, we divide the surface to be covered by the surface of each sheet
As we can't buy 0,75 of a sheet, we round it up, i.e. we will need 25 sheets of plywood for subflooring.