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Published byLesly Lammey Modified over 2 years ago

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Fractions: Multiplying by more interesting fractions – and then DIVIDING by them. (Part Two)

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To find 1/8 of something, we divide that thing by 8. What if we wanted to know what 3/8 of something was?

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You’d be doing the same thing 3 times, so you would multiply by 3.

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(72 is a whole number – so it’s all in one group. 72 ÷1 is… 72.) “John has saved 5/6 of the 72 dollars he needs. How much has he saved? How much does he still need to save?” … Of means multiply, so this problem will look like this:

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Divide 72 by 6… then multiply by 5. 60

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What would that *look* like?

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If I divide that 72 dollars into 6 groups (as the denominator tells me to do), then each “1/6” will have 12 dollars. 6/6 of 72 will be 6 out of six… the whole thing. 6/6 is 1… 1 x 72 is 72.

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5/6 is going to be most of the money… 5 x 12 or 60 dollars. 5/6 is going to be most of the money… 5 x 12 or 60 dollars.

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Of means multiply… BUT if you multiply by a fraction that’s smaller than one, you don’t have your “whole thing” – so your answer will be smaller. So… 5/6 of 72 is the same as 1/6 of 72…which is 12… times 5, which is 60.

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It would be the same as ½. How much would 3/6 of 72 be?

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We could draw every fraction to check that out… or we could practice division… but if the numerator is half of the denominator, then the fraction is equivalent to ½.

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Which of these fractions are the same as a half?

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How could you tell which ones were *more* than a half?

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Dividing by fractions But enough with the multiplying, already… time to cover a division problem that is much easier to understand when you can see it.

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WATCH YOUR LANGUAGE!!!!!!! Divided by doesn’t mean divided into… doesn’t me a fraction of. If I say 6 ÷ 6, my answer will be the number of times I can get six away from six, which is ONE WHOLE TIME. As a fraction, that would look like this:

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6 ÷ 2 is 3 6 ÷ 3 = 2

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6 / 6 is ONE.

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What happens if I divide 6 by ½ ?

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How many *halves* can I get out of six whole oranges? After all, I’m just not that hungry…

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No matter how I slice ‘em (as long as they’re in half), I’ll get 12.

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In math, this looks like In math, this looks like

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And the way to get this without drawing everything is:

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“Copy, Change, Flip” is the recipe…

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The concept is that if I divide by a big ol’ whole number, I get smaller… but if I divide by a little piece, I can spread things out further so I get bigger.

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You try drawing 5 ÷ 1/3

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Now to mix ‘em up… watch that language ½ of 50 ___ 50 ÷ ½ = ___ 1/3 of 18 = ____ 18 ÷ 1/3 = ____

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Next stop… adding fractions… but don’t forget – OF means MULTIPLY

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