This is one of those deceptively simple ideas that accomplishes much more than one might guess. There are 96, 4" x 4" double-sided cards in each set. In Single Digits each has four single-digit numbers. In Double Digits, there are combinations of single- and double-digit numbers. The challenge is to combine those four numbers using addition, subtraction, multiplication, and division to make a total of 24. Since the cards are divided into three groups, from easy to difficult, children as young as nine (approximately) can play using the easy cards of the Single Digit game, while adults will find some of the difficult ones in either game very challenging. The beauty of such a game is the amount of mental math that takes place to find the correct solution.
Those who need easier levels can work with the "Primer" editions: Add/Subtract Primer and Multiply/Divide Primer.
Advanced elementary students should try the Fractions/Decimals Edition, suggested for ages 11 and up. As with the Single Digit edition, there are three levels of difficulty, but I think everyone will want to start at the easiest level to figure out how to play. The cards have from one to three fractions or decimals per card as well as single-digit numbers. Otherwise, play is just like that for the Single-Digit game.
Algebra/Exponents is a challenging game for students at least junior high level or older. It, too, offers three levels of difficulty, and I think most adults would struggle with the second and third levels. For example: given the four terms: 4, 3, 7, and 2(x+y), add, subtract, multiply, or divide using all four terms to come up with x and y values that produce an answer of 24. When you are working at learning the subject already, it is not as difficult as it looks at first. Also, hints for common patterns help us out a bit.
Games can be played in groups or solo. This game has proven so popular that there are 24 Challenge tournaments (see their website for details) across the country. This is one of the few products that I will say that I highly recommend.