Pairs for big classes

The game is just concentration, I have 20 pieces of A4 colored construction paper, numbered 1-20. I will have 10 pictures repeated twice on the opposite side. I put up the 20 pieces of A4 on the blackboard and then divide the kids into 2 groups.  Bring up one student from each team, they janken and the winner has to chose two numbers and say them in English (ie 5 and 17 please). I turn over those 2 numbers, if they are the same, the team gets 1 point, then the other student picks two numbers. Keep bringing up pairs of students and playing the games until all pictures have been matched up.