Max's deck also has 40 blue cards, and for every 5 blue cards in his deck there are 9 red cards. His blue cards can be divided exactly into 8 groups (8 times 5 is 40), so he should also be able to divide his red cards into exactly 8 groups of 9 cards each. So there are 72 red cards in his deck (8 times 9 is 72).