In the previous lessons, you learned that a fraction is part of a whole. Fractions show how much you have of something, like 1/2 of a tank of gas or 1/3 of a cup of water.
In real life, you might need to add or subtract fractions. For example, have you ever walked 1/2 of a mile to work and then walked another 1/2 mile back? Or drained 1/4 of a quart of gas from a gas tank that had 3/4 of a quart in it? You probably didn't think about it at the time, but these are examples of adding and subtracting fractions.
Click through the slideshow to learn how to set up addition and subtraction problems with fractions.
Let's imagine that a cake recipe tells you to add 3/5 of a cup of oil to the batter.
You also need 1/5 of a cup of oil to grease the pan. To see how much oil you'll need total, you can add these fractions together.
When you add fractions, you just add the top numbers, or numerators.
That's because the bottom numbers, or denominators, show how many parts would make a whole.
We don't want to change how many parts make a whole cup (5). We just want to find out how many parts we need total.
So we only need to add the numerators of our fractions.
We can stack the fractions so the numerators are lined up. This will make it easier to add them.
And that's all we have to do to set up an addition example with fractions. Our fractions are now ready to be added.
We'll do the same thing to set up a subtraction example. Let's say you had 3/4 of a tank of gas when you got to work.
If you use 1/4 of a tank to drive home, how much will you have left? We can subtract these fractions to find out.
Just like when we added, we'll stack our fractions to keep the numerators lined up.
This is because we want to subtract 1 part from 3 parts.
Now that our example is set up, we're ready to subtract!
Try setting up these addition and subtraction problems with fractions. Don't try solving them yet!
You run 4/10 of a mile in the morning. Later, you run for 3/10 of a mile.
You had 7/8 of a stick of butter and used 2/8 of the stick while cooking dinner.
Your gas tank is 2/5 full, and you put in another 2/5 of a tank.