This is a game called “If this is the answer, what is the question?”
Give children individual whiteboards, then say a number. Encourage them to find as many ways to make the number as possible with as many operations. So if you say 20, they could go for an easy 10+10 or a more difficult 100 divided by 5.
This can be differentiated by giving them fractions or decimals, or more obscure numbers. It is also differentiated by outcome.