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Maths Tutorials: How to subtract fractions with different denominators

Subtracting one fraction from another while both have different denominators without the help of a calculator is a pretty much impossible task. Unless you happen to know the exact numeric value of the fractions. Luckily enough though there is a method that can be applied to carry out the subtraction anyway.

The key is to find the smallest common denominator, and converting both fractions accordingly. So for instance, let's say we have the following scenario:

3/4 - 1/5

The smallest common denominator between these two fractions is 20. It just so happens that this number could also be reached by multiplying 4 times 5. If you have no common denominator in sight, and can't seem to find one, multiplying the denominators by one another is often the quickest way to find one.

So, using 20 as the common denominator will turn the above calculation into this:

3/4 - 1/5 = 15/20 - 4/20

And all of a sudden we are looking at a calculation that is actually solvable. 15 minus 4 after all is not exactly rocket science. This means the result is 11/20, which cannot be rounded down any further, and is therefore our end result.

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