You are assigned 32 math exercises for homework. you complete 87.5% of these before dinner. how many do you have left
to do after dinner?

So we need to find 87.5% of 32 then subtract from 32 to find how many we have left.  In math

"is" means equals (=)
"of" means times (*)

87.5% of 32 is
87.5% * 32 =
.875 * 32 =
28

So she finished 28 problems before dinner.  Now she has

32 - 28 = 4 left