Prepare "decks" of ten cards for each group of students.
The cards should have the numbers 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9 on them.
Put the cards face down.

In round one, have one player pick seven of the cards and turn them face up.
Each player in the group must select six of those cards to form two three-digit numbers that will give him the largest sum.   One card will be a discard.
The player with the largest sum scores a point for that round.

For round 2, have another player choose seven of the ten cards.
This time, each player makes three two-digit numbers with one discard.
The goal is to add two of those numbers and subtract the third to get closest to a total of 50.
Again, the player who is closest to 50 scores a point for that round.

For round 3, have another player choose four of the ten cards.
This time, each player makes two two-digit numbers with no discards.
The goal is to find the highest product of the two numbers.
Again, the player with the largest product scores a point for that round.
Hint for Teacher: The smaller the difference, the larger the product.