Answer
$n(A) = 50$
Work Step by Step
Given:
$n(A∪B)= 60 , n(A∩B) = 40, n(A) = n(B)$
Use the formula $n(A \cup B) = n(A) + n(B) - n(A\cap B)$ to obtain:
$n(A \cup B) = n(A) + n(B) - n(A\cap B)
\\n(A \cup B) = n(A) + n(A) - n(A\cap B)
\\n(A \cup B) = 2[n(A)] - n(A \cap B)
\\60 = 2[n(A)] - 40
\\60+40=2[n(A)]
\\100=2[n(A)]
\\\frac{100}{2}=\frac{2[n(A)]}{2}
\\50=n(A)$
