See also the Counting Game for a simpler version of this.
Competitors pair up and hold just one hand behind their back. Choosing to show 0 to 5 digits. Both players reveal the number of fingers they are showing at a chosen signal. Whoever yells out the sum of the fingers wins the first round.
Round 2, multiply the fingers.
Round 3 is the difference between the two numbers.
Rotate through as many rounds as you want, but for the championship round (between the two survivors) have them square the sum of the two sets of fingers. For example, Person A shows three fingers, person B shows four, so the winning answer is 3+4 = 7 * 7=49