What is 10 plus 9 plus 1 minus 1 plus 2 minus 2 plus 3 minus 1 plus 4 minus 4?

21

It is 21.

(10+9)+(1−1)+(2−2)+(3−1)+(4−4) = 21

This is the same equation rewritten: (10+9)+(3−1) = 21
This can be done as 1-1, 2-2, 4-4 cancel themselves out.