Set up two fractions with x= the number of questions correct
x/20 = 85/100
This is to find how many questions the student got right, then we'll use the answer to find how many they got wrong.
First, cross multiply the numerators and denominators:
100x=85 times 20
100x=1700 Divide both sides by 100
x=17. We know they got 17 questions correct, which means they got 3 question wrong