# Working with Fractions

People don’t like fractions. I don’t know why. They’re difficult to begin with, I know, but a few simple rules will help you add, subtract, multiply and divide.

Adding and subtracting are usually the easiest sums, but not when it comes to fractions. If fractions have the same denominator (the number on the bottom), then you can simply add or subtract the second numerator from the first, eg 4/5 – 3/5 = 1/5. If not, it would be like adding apples and oranges.

They’re just not the same, so you first have to convert them into ‘pieces of fruit’ – or a common unit. The easiest way of doing that is by multiplying the denominators together. That guarantees that the new denominator is a multiple of both the others.

Once you’ve found the right denominator, you can multiply each numerator by the denominator from the other fraction (because whatever you do to the bottom of the fraction you have to do to the top), add or subtract them and then simplify and/or convert into a mixed number if necessary, eg 2/3 + 4/5 = (2 x 5 + 4 x 3) / (3 x 5) = (10 + 12) / 15 = 22/15 = 1 7/15.

1. Multiply the denominators together and write the answer down as the new denominator
2. Multiply the numerator of the first fraction by the denominator of the second and write the answer above the new denominator
3. Multiply the numerator of the second fraction by the denominator of the first and write the answer above the new denominator (after a plus or minus sign)
4. Add or subtract the numerators and write the answer over the new denominator
5. Simplify and/or turn into a mixed number if necessary

Note that you can often use a simpler method. If one of the denominators is a factor of the other, you can simply multiply the numerator and denominator of that fraction by 2, say, so that you get matching denominators, eg 1/5 + 7/10 = 2/10 + 7/10 = 9/10. This means fewer steps in the calculation and lower numbers, and that probably means less chance of getting it wrong.

### Sample Questions

1. 1/5 + 2/3
2. 3/8 + 11/12
3. 13/24 – 5/12
4. 7/8 – 3/4
5. 5/8 – 2/3

## Multiplication

This is the easiest thing to do with fractions. You simply have to multiply the numerators together, multiply the denominators together and then put one over the other, simplifying and/or converting into a mixed number if necessary, eg 2/3 x 4/5 = (2 x 4) / (3 x 5) = 8/15.

1. Multiply the numerators together
2. Multiply the denominators together
3. Put the result of Step 1 over the result of Step 2 in a fraction
4. Simplify and/or turn into a mixed number if necessary

1. 1/5 x 2/3
2. 7/12 x 3/8
3. 4/5 x 2/3
4. 4/9 x 3/4
5. 5/8 x 2/3

## Division

Dividing by a fraction must have seemed like a nightmare to early mathematicians, because nobody ever does it! That’s right. Nobody divides by a fraction, because it’s so much easier to multiply.

That’s because dividing by a fraction is the same as multiplying by the same fraction once it’s turned upside down, eg 2/3 ÷ 4/5 = 2/3 x 5/4 = (2 x 5) / (3 x 4) = 10/12 = 5/6. You can even cut out the middle step and simply multiply each numerator by the denominator from the other fraction, eg 2/3 ÷ 4/5 = (2 x 5) / (3 x 4) = 10/12 = 5/6.

1. Multiply the numerator of the first fraction by the denominator of the second
2. Multiply the numerator of the second fraction by the denominator of the first
3. Put the result of Step 1 over the result of Step 2 in a fraction
4. Simplify and/or turn into a mixed number if necessary

Note: In some cases, it’s possible to divide one numerator by the other and one denominator by the other, but the answers must both be whole numbers for it to work, eg 8/9 ÷ 2/3 = (8 ÷ 2) / (9 ÷ 3) = 4/3 or 1 1/3, but 7/9 ÷ 4/5 doesn’t work because 4 doesn’t go into 7 evenly and 5 doesn’t go into 9 evenly.

The advantage of the second method is that you don’t have to spend so much time simplifying the resulting fraction as the numbers are divided rather than multiplied, which makes them smaller. If we cross-multiplied using the standard method, we’d end up with 24/18 rather than 4/3. However, the benefit of the standard method is that it always works!

1. 1/5 ÷ 2/3
2. 2/7 ÷ 3/5
3. 4/7 ÷ 2/3
4. 7/8 ÷ 3/4
5. 5/6 ÷ 2/3

## Simplifying Fractions

One way of simplifying fractions is to divide by the lowest possible prime number over and over again, but that takes forever! It’s much simpler to divide by the Highest Common Factor (or HCF), which is either the numerator itself or half of it or a third of it etc:

1. If possible, divide both the numerator and the denominator by the numerator. If that works, you’ll end up with a ‘unit fraction’ (in other words, 1 over something) that can’t be simplified any more, eg 7/14 = 1/2 because 7 ÷ 7 = 1 and 14 ÷ 7 = 2.
2. If the numerator doesn’t go into the denominator, try the smallest fraction of the numerator (usually a half or a third) and then try to divide the denominator by the result, eg 24/36 = 2/3 because half of 24 is 12, and 36 ÷ 12 = 3.
3. If that doesn’t work, keep repeating Step 2 until you find the answer, eg 24/30 = 4/5 because a quarter of 24 is 6, and 30 ÷ 6 = 5 (and a half and a third of 24 don’t go into 30).

### Sample Questions

1. Simplify 14/28
2. Simplify 8/24
3. Simplify 30/50
4. Simplify 27/36
5. Simplify 45/72

## Turning Improper Fractions into Mixed Numbers

To turn an improper fraction into a mixed number, simply divide the numerator by the denominator to find the whole number and then put the remainder over the original denominator and simplify if necessary, eg 9/6 = 1 3/6 = 1 1/2.

1. Divide the numerator by the denominator
2. Write down the answer to Step 1 as a whole number
3. Put any remainder into a new fraction as the numerator, using the original denominator
4. Simplify the fraction if necessary

### Sample Questions

1. What is 22/7 as a mixed number?
2. What is 16/5 as a mixed number?
3. What is 8/3 as a mixed number?
4. What is 18/8 as a mixed number?
5. What is 13/6 as a mixed number?

## Turning Mixed Numbers into Improper Fractions

To turn a mixed number into an improper fraction, multiply the whole number by the denominator of the fraction and add the existing numerator to get the new numerator while keeping the same denominator, eg 2 2/5 = (10 + 2)/5 = 12/5.

1. Multiply the whole number by the denominator of the fraction
2. Add the answer to the existing numerator to get the new numerator
3. Write the answer over the original numerator
4. Simplify if necessary

### Sample Questions

1. What is 2 2/7 as an improper fraction?
2. What is 3 2/3 as an improper fraction?
3. What is 4 1/4 as an improper fraction?
4. What is 5 1/5 as an improper fraction?
5. What is 3 2/9 as an improper fraction?

There you go. Easy peasy lemon squeezy!