We are ready now to combine what we have learned about directed numbers with our knowledge of fractions. Just like we saw with integers, we can order and compare fractions and perform addition and subtraction of fractions using the number line.
On the number line below, each tick is labelled with a multiple of the fraction \dfrac15. We can see that the point furthest to the left is plotted at the fraction -\dfrac35, and the point furthest to the right is plotted at the fraction \dfrac65.
It is common to see number lines where only the integers are labelled, with ticks between each integer that represent a fraction of one whole.
We can find the denominator of the fraction using the number of ticks between each integer. To go from 0 to 1 we need to move up 3 ticks, so each tick represents \dfrac13. To find the numerator of the fraction we can count the ticks from 0 to the point, which gives 8. This means that the point is plotted at the fraction \dfrac83.
Another way to identify the fraction is to see that the point is two thirds to the right of the integer 2. So it lies on the number 2+\dfrac23, which we can write as the mixed number 2\dfrac23.
Use the following applet to plot fractions on a number line.
Follow the directions in the applet and click New Numbers to explore more fractions on the number line.
The number of spaces between the numbers is the denominator and the spaces between the point and 0 is the numerator.
Fractions and mixed numerals can be plotted on the number line.
Where is the point plotted on the number line?
The number of spaces between the numbers is the denominator and the spaces between the point and 0 is the numerator of the plotted fraction on a number line.
If we look at any two integers, it is simple to see which is greater. But if we are given two fractions, it can be less obvious to see which is greater if the fractions do not have the same denominator.
Now let's compare -\dfrac13 and -\dfrac25. In this case we need to rewrite both fractions so that they have a common denominator. Let's choose a denominator of 15, which is the lowest common denominator of the two fractions. We rewrite -\dfrac13 as -\dfrac{1\times 5}{3\times 5}=-\dfrac{5}{15} and we rewrite -\dfrac25 as -\dfrac{2\times 3}{5\times 3}=-\dfrac{6}{15}.
Which number is greatest?
We need to find the lowest common denominator of the fractions to compare them.
Adding and subtracting positive and negative fractions works in the same way as adding and subtracting positive and negative integers.
Find the value of \,2\dfrac{2}{9}-\left(-\dfrac{5}{9}\right).
Addition and subtraction of directed fractions can be done the same way as the addition and subtraction with positive and negative integers.
Adjacent opposite signs results in a negative sign when combined.
Adjacent same signs results in a positive sign when combined.