Be a human calculator.
Techniques to add without paper.
Ramanujan could perform complex calculations instantly.
Break numbers apart: 28 is just 20 + 8.
Plain English: Your brain likes easy numbers (ending in 0). If you have to add 19, just add 20 and subtract 1.
Real-world example: Checking if you have enough cash at the checkout line before you get embarrassed.
Forgetting to subtract the extra bit you added to make the number round.