The exercise is apparently directed toward getting you to group the positive integers together and the negative integers together before you work out the difference. Doing that gives you ...
a) (10 +5) -19
b) = 15 -19
= -4
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Personally, I wouldn't do that <em>for this problem</em>. If you do the arithmetic in order, it seems easier to me because you're only doing single-digit arithmetic after the first sum.
= (10 -19) +5
= -9 +5
= -4
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The idea is that there are different stratgies that you can employ to simplify your work and reduce errors. Grouping positive and negative numbers together is one such strategy. Arranging the sums so they almost cancel is another strategy you can use.
