TenMarks teaches you how to subtract using clock fractions.
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Learn about Subtracting with Clock Fractions In this lesson, let’s learn how to subtract two fractions using the clock fraction method. All the clock fraction method does is works on fractions that have a denominator that is 2, 3, 5, 6, 10, 12, 15, 20, 30 or 60. Defensibly any factors of 60. Here’s how here it works. If we want to subtract two fractions, let’s say 1/3 and from that we need to take away 1/6 or 1/3-1/6. We can represent both of these as clock fractions. What do we do? Well we know that in a clock, one turn is 60 minutes, which is one hour. We’re talking about the big hand. So if one turn is 60 minutes, if I wanted to do 1/3 or 1/3 hour, what is 1/3 hour? 1/3 of an hour is this much, which is the same as 20 minutes. So 1/3 is the same as 20 minutes when we convert that to an hour. Similarly, 1/6 of an hour is ten minutes. So ten minutes can be represented using two of these gradients. So each gradient has five minutes if you remember. So this is 12—1, 2, 3. So I have the big needle move from 2 to 3, it’s gone 5 minutes so 10 minutes is two of these. So if I have something that is 1/3 and from that I want to take away 1/6. Instead of 1/3 I could say, I have 20 minutes and I want to take away 10 minutes. What will we be left with? If from 20 minutes, we took away 10 minutes, we’re left with 10 minutes. 20 minutes-10 minutes=10 minutes. What is 10 minutes? Well 10 minutes is 1/6 hour. So what do we have? We can say, 1/3 and from that if I subtract 1/6, I'm left with 1/6. And we did that by just converting each of these fractions into an hour clock and we can do that for any fractions that have these denominators.

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