Mathematics
A perfect number is a natural number that is equal to the sum of all of its proper divisors (divisors that are less than the number itself). For example, 6 is the smallest perfect number because its proper divisors, 1, 2, and 3, add up to exactly 6.
Consider all pairs of natural numbers that add to 9, which pair has the largest possible Least Common Multiple?
Triangular numbers and trapezoidal numbers are both natural numbers that can be expressed as the sum of more than one consecutive natural numbers.
Some numbers can be expressed as the sum of two perfect squares...
Sliding numbered blocks in a square grid to put them back in numerical order.
Constructing an unsolvable setup in a colour sorting game.
Lights connected in a string or a loop, each attached to a switch that will change the state of the light itself and its neighbours when pressed.
Connecting letters on the left to their matching letters on the right without crossing any lines.
The card game Seven is one where players take turn extending the numerical sequences of cards in their respective suits.
Irish Snap is a multiplayer card game using the standard deck of 52 playing cards where each player takes turns laying down one face up card each while counting. I.e. the first person says “ace” while laying down the first card; then the second person says “two” while laying down the next card. This process is repeated until the card that has just been laid down matches the number the person just called. When such a match happens, all players race to slap a hand on top of the pile of cards, and the slowest person has to take the pile into their hand. The first person to empty their hand wins.