The 24 Game is manufactured by Suntex International in 1993. The object of the game is simple. From four numbers on the card, make 24. You can multiply, add, divide, or subtract to do this. Each number on the card must be used once, and only once. Rather than just show examples on how this works, have a look at this.
There are a wide variety of variations of the 24 game, like decimals, single digits, algebra, double digits, and others.
The cards are a solid blue square, 4 inches across, with a yellow circle inscribed in it. In the center of the card, there's a red or white square, with four red polygons extending outward, dividing the circle in 4. In each of these divisions, a number is posted.
Each card also has a number of dots in each corner: 1 white dot for easy, 2 red dots for medium, and 3 yellow dots for difficult. I find that the easy cards are a matter of reflexes, while the difficult cards keep the brain grinding for days.
I'll post some samples from sets other than Single Digits. Just be aware that these samples are not part of my contest.
Double Digits: 10 11 6 2
Fractions: 3 3/4 3 6
Algebra: 9 3 y+2 3