Answer:
9 sets of glasses
Explanation:
If Kevin needs 491 glasses and he already has 333 glasses, this means he only needs 158 glasses. If each case has 18 glasses you can make the equation 18x = 158 and to get x by itself you would divide 158 by 18 to get 8.7 and since you have to buy the cases as sets of 18, you would need 9 sets.