Do you use use your iPhone as a calculator? I do and I’m always slipping up, pressing the wrong key on the Calculator app. It can be frustrating to start my sums back at the beginning. However, you can swipe across the display to delete the last number you tapped. Keep swiping away to delete any number of digits.
For more number fun you can also ask Siri to solve calculations for you. Just hold down the home button and ask. Although for Siri, you will need a network connection.