The average of the two numbers is -39/2 = -19.5. The smaller number is 0.5 less than this, -20, and the larger number is 0.5 more than -19.5, so is -19.
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Comment on the problem and solution
I find that working consecutive integer problems is often simplified by working with the average value of those integers:
the average of two consecutive odd integers is the even integer between them
the average of two consecutive even integers is the odd integer between them
the average of an odd number of integers of the same type (consecutive, consecutive odd, consecutive even) is the middle one
The average of an even number of consecutive integers is the "half" number between the middle two (as in this problem).
