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why do we round up or down?

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3 Answers

Since your son is in elementary school, the simpliest answer is: Rounding up or down helps make is easier & quicker for people to understand the number.  By rounding up or down it lets people know *about* what the number is & it helps when your not exactly sure the number is accurate. 

Example: I have *about* 6 cups left of milk.     OR      I have 5.87 cups left of milk.

Which sentence is easier & quicker for you to understand?

If you measured the milk yourself, would you rather be graded on pouring *about* 6 cups or 5.87 cups.  If you answered *about* 6 cups, you would have the same answer as most scientist or reporters because they wouldn't want to get in trouble either if some of their measuring was not exact!  


Of the three explanations, I think your was the best.

We also round up or down to show a range. For instance, if I was measuring snakes and I measured them as follows:

20 inches, 15 inches, 6 inches, 25 inches, and 34 inches

I could say "The snakes I measured were about 5 inches to 35 inches long." It gives the idea (an estimation) of the general size, without having to list each specifically.


Roger - Hello!

There are a couple of reasons to do this.  One reason is that whole numbers are easier to work with (add, subtract, multiply, and divide) instead of decimals.  If we get rid of the decimal point by rounding, we don't have to worry about putting the decimal in the wrong place!

Another reason is to allow for errors when we know an answer we have isn't 100% accuracte.  This happens when social scientists do surveys (for example).  They might send out so many surveys that they're afraid they might have lost count!  So, they round up (usually) to account for miscounted surveys.

I hope this helps!

Jeff Stickler, M.Ed.

(Indiana, USA)