Not sure if this is going to work, but, we can give it a go

i start with a number, 21 for example, the next person must make that number using all 4 signs (x,+,-, / )

They can use any numbers they want and can use the signs more than once aslong as they use all four

21

5+7/2x3-2+5 = 21

And so on

Starting number = 37