About Hidden Card or X-Ray Vision

What's the math in this game?

In this game, children can use a counting strategy called 'counting on'. Counting on is when you count up from a number other than one. Encourage children to “count on” from a face-up card to a hidden card. For example, they can count on from the face-up 3 card to the hidden 6 card: “3…4, 5, 6!”