You have to multiply 1/4 x 1/4 x 1/4 for the volume.
90-x*30=80
90-30x=80
90-90-30x=80-90
-30x=-10
Divide by -30 for -30x and -10
-30/-30x=-10/-30
x=1/3
f = 4/5
explanation
f(10) = 8
so you divide both sides by ten in order to isolate the variable
10f/10 8/10
you now have
f = 8/10
simplify 8/10 to get
f = 4/5
hopefully i explained it well :)