What is the average of all the numbers 1 to 1000?

500.5

Because:

1+2+3+.......998+999+1000

Add 1 to 1000 equaling 1001.

This makes a pair.

There are 500 more of these so 500 pairs.

So 1001x500= 500500

Finally divide it by 1000 because there are 1000 numbers. (1000-1=999+1=1000)

Concluding the answer to be 500.5

:)