What is the opposite of 6 on the number line:

The correct answer is: -6

The center of the number line is 0. It extends in both positive and negative directions.
So, the number 6 is at sixth place in positive direction from 0.
The number opposite to 6 will be 6 place in negative direction from zero.
The number which is at sixth place in negative direction from 0 is -6.
The right answer is -6.

For such questions, we should be careful about the counting proper intervals.