The app is doing mental arithmetic. The screen changes automatically. You don't have to enter the answer. You are just watching. Continue to subtract the number and continue to add the number. When it becomes less than 0, it changes to addition, and when it becomes larger than 100, it changes to subtraction. Tap the number in the center to start the calculation. Add or subtract the number on the upper right to this number. When you can calculate and want to proceed quickly, tap the paw pad. (If you want to calculate at your own pace, set a longer time and tap the paw pad to proceed. ) The number on the upper right is fixed, but you can tap the number to change it randomly between 3 and 9.