Simple Addition?

Find the value of 87-76+65-54+43-32+21-10.  This problem illustrates a couple of the most elementary problem solving strategies, that can be explained to the youngest students.  First, seeing a long computation should prompt you to consider an easier way to compute other than starting with the first number and proceeding to the right.  Second, subtracting is the same as adding the negation, so you can use the associative property of addition to add the numbers in any order.  In this case, you can compute 87-76, then 65-54, then 43-32, then 21-10.  Replacing the above expression with 11+11+11+11, then the computation is, as they say, trivial.  

(Source: MOEMS)

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