I got that|G|=40 and |Z(G)| contains an element of order 2. From Lagrange i know that the order of Z(G) must divide |G| and be a multiple of 2. I am able to do all the cases by the G/Z theorem accept for 1 case. This is the case where |Z(G)|=2. Then I get |G/Z(G)| =20, and I cant use one of the nice theorms like the 2p theorem or the p^2 theorem to get the isomorphism type. Does anyone have any ideas on what I should do?