All computations are to be done mentally. There is to be no writing on paper, on their hands, on the car, etc. Participants may write invisibly in the air and/or use their fingers and body parts. Talking is outlawed. Also, no calculators are allowed.
Read a list of computations slowly allowing everyone to make the calculations in their head.
When you are finished with all of the questions, ask if anyone would like a problem repeated.
Once you have finished a list of computations, tell everyone to write down their answer.
Have everyone read their answer out loud to see who all got the right answer.
10 / 2 … + 10 … * 2 … – 20
The answer would be 10.
These activities can be tailored to the age of the children and their math capabilities. If necessary, do one set of kids for one round, and another set of the children on a subsequent round.