number defective=4
number of non-defective=total-defective
=20-4
=16
the questions asked for 1 defective
so we choose only 1 defective from 4 defective items
4C1
and for 1 non-defective
we choose only 1 non-defective from 16 non-defective items
16C1
we choose 2 items from 20 items
so 20C2
answer:
probability= [(4C1) * (16C1)] /(20C2)
=32/95