Dominic S. answered • 09/14/15
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42.8 is pretty close to 40, 5.5 pretty close to 5, and 40 is easy to divide by 5, making them compatible numbers. We're also moving in the same direction (down in each case), which is a good idea when estimating quotients or differences. (When estimating products or sums, it's better to move in opposite directions). 40/5 is 8, so 42.8/5.5 should be somewhere close to 8.
