HEY GUYS I NEED HELP IN MY ALGEBRA somebody smart over there please :)))) you guys thanks so much love ya?

Gwen purchased 28 items, spending $108. Ballons cost $3 each, bags of candy cost $4 each, and caps cost $5 each. As student walked by, she gave each student one of each. However, after 7 students, she found out that she had run out of one of the items. How many of each did Gwen purchase.