3. In the sum $(1-2-3+4)+(5-6-7+8)+\ldots+(2009-2010-2011+2012)+$ (2013-2014-2015), all numbers that end in 0 were crossed out. Find the value of the resulting expression.
3. Answer: -3026. It is easy to see that in all parentheses except the last one, the sum is zero. Then the entire sum before cancellation was equal to -2016. At the same time, numbers that end in 0 and are divisible by 4 were included in it with a plus, while numbers that end in 0 and are not divisible by 4 were included with a minus. Therefore, to find the final answer, the first numbers should be subtracted, and the second ones should be added. Then the final sum is -2016 + $(20+40+60+\ldots+2000)-(10+30+50+\ldots+2010)=-2016+1000-2010=-3026$.

Criteria: full solution with explanation - 7 points. Correct calculations are provided, but comments are missing - 5 points. The sum is written correctly, but calculated with an error - 3 points. Only the answer - 0 points.