Let the cardinality of sets A and B be represented by n(A) and n(B), respectively.

According to the question,

$\rm n(A) = 40, n(B) = 50, n(A \cup B) = 60$

To find: $\rm n (A \cap B) =?$

We use the formula for the union of two sets to find the cardinality of the intersection of those sets, we get,

$\rm n(A \cup B) = n(A) + n(B) - n(A \cap B)$

$\rm or, n (A \cap B) = n(A) + n(B) - n(A \cup B)$

$\rm or, n(A \cap B) = 40 + 50 - 60$

$\rm \therefore n(A \cap B) = 30$

Hence, 30 elements. form the set $\rm (A \cap B)$.