40-39+38-37+...+24-23+22-2 1

=(40-39)+(38-37)+......+(24-23)+(22-2 1)

= 1+ 1+....+ 1+ 1

= 10

Subtraction calculation:

If you subtract a number from the sum of several numbers, you can choose any addend to subtract this number and add it to the rest of the addends. For example: (35+17+29)-25 = 35-25+17+29 = 56.

If you subtract several numbers from a number in succession, you can add all the subtractions first, and then subtract the sum of the subtractions from the minuend. For example: 276-115-85 = 276-(115+85) = 76.