You want to take the higher of the two numbers (in your case, 15 degrees C) and subtract the smaller (-13 C) from it. Mathematically, 15 - (-13) = 28 degrees C. So you are correct in your answer. This is an example of where subtracting does not always make the answer smaller, because subtracting a negative is basically the same as adding a positive!

If you'd like more examples, I'm including a link to purplemath.com which shows some good examples of adding and subtracting integers.

Consider drawing a number line with the two values (15 and -13) on it. What is the distance between those two points? It's 28.
Source(s):
http://www.purplemath.com/modules/negative2.htm

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