Concentration

This is a simplified single-player version of the card game concentration.
Only 20 cards are used in this version rather than the entire card deck.

The game starts with the 20 cards face-down on the playing field. The player selects 2 cards and these cards are turned over to reveal their face values. If the face values of the cards match (the suit does not matter), then the 2 cards are removed from the playing field, otherwise they are turned face-down in their original positions. The player must remember the positions and face values of all the cards as they are turned over so that they can match pairs as the game progresses. Play continues until the playing field is cleared.

The game is won when the playing field is cleared. The game is scored by the elapsed time from the start of the game until the playing field is cleared. The high score is stored and displayed on the main screen.