Last week we learnt how to add fractions. Today, we are going to learn how to subtract fractions.
Have a look at the in focus question below.
How many pizzas are there?
What fractions do you think we are going to be subtracting? Is it possible to subtract these?
We know that if ¼ of the pizza has been eaten, we will be left with ¾.
How do we subtract 1/8 from ¾?
If the denominators were the same, we would just need to subtract the numerators.
3/4 - 1/4 = 2/4.
The denominators have stayed the same and we have just taken away the numerators.
In the question above we know that we need to make the denominators the same to make it easier to subtract. This is what we have to do if the bottom numbers are not the same.
At the moment, we have ¾ of a pizza left and we need to subtract an 1/8 from it.
How can we convert one of these fractions so the denominator is the same? Can you spot a relationship between the numbers 4 and 8?
That’s right, in the fraction 3/4 we can multiply the denominator 4 x 2 to make 8 so if we make the same changes to the numerator too we know that 3 x 2 = 6 so we have now made the fraction 6/8.
You can see that we have converted the fraction 3/4 to 6/8 by multiplying both the top and bottom number by 2.
If we now take 1/8 from 6/8 we have 5/8 left.
Now, have a look at Q2 above.
If we take away 3/8 away from 2 whole ones how many are we left with?
How many eighths make a whole one? That's right. 8/8 so how many eighths in 2 whole ones?
Well done, we would have 13/8 which converted to a mixed fraction would be ...
1 whole one and 5/8!
Now try Q3 on your own.
What does difference mean?
Are the denominators the same? What could you do to help you?
Now try the Guided Practice section above then complete worksheet 12 which will be sent to you on Seesaw.