Partners enter the same number into both
calculators. Then one partner secretly changes the number adding
or subtracting some number, n, to or from the original number--and
shows the partner the new number that appears in the display.
The partner then tries to guess what was done to the original number.
Example:
Both calculators are set to display
17. Then one partner secretly changes the display by pressing [+]
13 [=], and shows the result (30). The other says, "You added
13."
