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Answer:
20
Step-by-step explanation:
we know,
n(aUb)=n(a)+n(b)-n(a∩b)
so,
80=40+60-n(a∩b)
or, 80-(40+60)=-n(a∩b)
or, -n(a∩b)=80-100
or, -n(a∩b)=-20
or, n(a∩b)=20