1. Take any calendar. Tell your friend to choose 4 days that form a square like the four to the right. Your friend should tell you only the sum of the four days, and you can tell her what the four days are. 2. Let's pretend that the 4 numbers that the person chose were the highlighted ones here - 18, 19, 25, and 26. She adds up the four numbers and tells you only that the sum is 88. 3. Express the sum given by your friend using algebra for finding the dates. 4. Solve the equation created to find out the dates of the calendar.