18) 3 parliamentary committees have been established, each consisting of 10 members. We know that no parliamentarian is a member of all three committees simultaneously. State the minimum number of people involved in the 3 committees.
(A) 10
(B) 15
(C) 20
(D) 25
(E) 29.
18) The answer is (B)

Each parliamentarian can ensure a maximum of 2 presences, while the number of presences required is $3 \times 10=30$. Therefore, at least 15 parliamentarians are needed; 15 is indeed the minimum number, as it can be achieved with the following combination: 5 parliamentarians who are members of the first and second committees, 5 members of the first and third committees, and 5 members of the second and third committees.