Answer: 10
Step-by-step explanation: The easiest way to learn how to understand these type of questions is through a number line. As you can see on the one I drew, there are 10 numbers between -1 and 9. Another way to solve this is see how far away your negative number is from zero. Also solve how far away from zero the positive number is. In your problem, you have 1 and 9. 1+9 is 10, which is also what was given from the number line.
Let's say you want to find the distance between -5 and 6. -5 is 5 away from zero, and 6 is 6 away from zero. 5+6 is 11, so the distance between the two is 11.
Please let me know if this helps! I can provide more examples until it makes sense
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