Assume there are no two such elements. Let A1,…,A22 be the initial sets, ∣Ai∣=5, i=1,…,22.
Put S=⋃i=122Ai and a(x,y)=∣{i∣{x,y}⊂Ai}∣ for x,y∈S.
Assume there are five sets that contain x and y, without loss of generality, A1,A2,A3,A4,A5. Then a set Aj with {x,y}⊂Aj must contain an element from Ai∖{x,y} for i=1,2,3,4,5. Hence, {1,2}∩Aj=∅ and a contradiction ∣A1∩Aj∣=2 follows.
Consequently, we can assume that a(x,y)≤4 for all x,y∈S.
There are exactly 10 unordered pairs contained in A1, and every Ai, i=2,3,…,22, contains exactly one of them. Hence, one of the pairs is contained in A1 and three other sets, i.e. there are x,y∈S with a(x,y)=4.
Without loss of generality, we assume that x=1,y=2 and that the four sets containing 1 and 2 are:
A1={1,2,3,a,b},A2={1,2,4,c,d},A3={1,2,5,e,f},A4={1,2,6,g,h}.
Furthermore, we can assume that A5={1,3,4,5,6}. To meet the condition ∣Ai∩Aj∣=2 for i=1,2,3,4 every Aj with j>4 must contain either 1 or 2. Assume that 1∈Aj for j=5,6,…,14. Each of the nine sets Aj, j=6,…,14, must contain 3,4,5, or 6 because of ∣A5∩Aj∣=2. Hence, there is an x∈{3,4,5,6} with a(1,x)≥5, a contradiction. Consequently, at most nine of the sets Aj, j=5,…,22, contain 1. Analogously, at most nine of the sets Aj, j=5,…,22, contain 2. It follows that exactly nine of the sets Aj, j=5,…,22, contain 1, without loss of generality, the sets A5,…,A13.
Taking into account that a(x,y)≤4 for all x,y∈S and ∣Ai∩Aj∣=2 for 1≤i<j≤13, without loss of generality, we can assume
A6={1,3,c,e,g},A7={1,3,d,f,h},A8={1,4,a,e,h},
A9={1,4,b,f,g},A10={1,5,a,d,g},A11={1,5,b,c,h},
and there is no proper choice left for A12.