In class A, there are 40/100 * 80 = <<40/100*80=32>>32 students.
In class B, the number of students is less by 21, which means class B has 32 - 21 = <<32-21=11>>11 students.
That means 80 - 32 - 11 = <<80-32-11=37>>37 students are in class C.
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