If there are 30 students in Ms. Leech's class, and the number of boys is 10, the number of girls in Ms. Leech class is 30-10=<<30-10=20>>20
If each boy brought 5 cups for the science activity, the total number of cups brought by the boys is 5*10=<<5*10=50>>50 cups.
The number of cups that the girls brought is 90-50=<<90-50=40>>40 of the total number.
Since the number of girls in the class is 20, each girl brought 40/20=2 cups each.
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