I read this as set A has 25 elements and set B has 15 elements.
(a) A∪B has a maximum of 25+15=40 elements meaning that B and A have no elements in common. Therefore max n(A∪B)=40.
(b) If B is a subset of A, min n(A∪B)=25.
(c) max n(A∩B)=15, if B is a subset of A.
(d) min n(A∩B)=0, if A and B have no common element.