TenMarks teaches you how to add using clock fractions.
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Learn about Adding with Clock Fractions In this lesson we’re going to use Clock Fractions, an interesting way to add two different fractions. Now clock fractions can be used for any denominators, any fractions that have the denominators as 2, 3, 5, 6, 10, 12, 15, 20, 30 or 60. Essentially we know that in a clock, one hour equals 60 minutes. So we can take one of the clocks and if the big hand goes from the top all the way down, that is 60 minutes. So what is 1/3 if it went 1/3 of the hour, how far did it go? What’s 1/3 of an hour? Well 1/3 hour is 20 minutes. So I can model 1/3 of a fraction or fraction 1/3 is the same as 20 minutes. So I drew down the clock. Then we take 1/6 of an hour. 1/6 of an hour is what, 10 minutes. So we start from 20 and we block of another 10 minutes. This is 1/6 which is 10 minutes. Now that I have one clock showing the 1/3 of an hour which is 20 minutes, the next one showing me 10 minutes represented. I can simply add these two to get—what do I get? Well I get 30 minutes. 20 minutes plus 10 minutes is 30 minutes. Well what percentage or what fraction of the clock is this? Well I can see this is half. So what do I get? 1/3 and 1/6 gives me ½ hour or a fraction called ½. See how simple it was? So if the fractions have denominators 3, 6 both work. I can model—convert the fractions using a clock. This is 20 minutes, this is 10 minutes. 20 minutes added to 10 minutes is 30 minutes which is half an hour. Half hour is what we were looking for.

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