Answer:
25 sheets of plywood will be needed
Step-by-step explanation:
Each sheet of plywood measures [tex]4ft*8ft=32ft^{2}[/tex].
We have to cover the surface of the house, minus the surface of the hole of the stairs.
[tex]Surface _{to cover} = Surface _{house} -Surface _{hole}= (28ft*30ft)-(12ft*4ft)= 840ft^{2}-48ft^{2}=792ft^{2}[/tex]
To know how many sheets will be needed, we divide the surface to be covered by the surface of each sheet
[tex]Sheets_{needed} =\frac{Surface _{house}}{Surface _{sheet}} =\frac{792ft^{2}}{32\frac{ft^{2}}{sheet} } = 24,75 sheets[/tex]
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.