Answer :
possibilities
20,21,22,23,24,25,26,27,28
2 remove all even
21,23,25,27
3 rules out 21 and 27
23,25
5 pulls out 25
23 is the answer
20,21,22,23,24,25,26,27,28
2 remove all even
21,23,25,27
3 rules out 21 and 27
23,25
5 pulls out 25
23 is the answer