Cross Multiplying Fractions – Don’t FEAR the FRACTION
Hey guys! Welcome to this video on how to cross multiply fractions.
When cross multiplying fractions, the name sort of hints at how this is actually done.
You literally multiply across. Let’s say you have two fractions that are set equal to each other: a/b=c/d
Well, to cross multiply them you multiply the numerator in the first fraction by the denominator in the second fraction, then write that number down. Then you multiply the numerator of the second fraction by the denominator of the first fraction. Here is what that looks like.
The reason we cross multiply fractions is to compare them. Cross multiplying fractions tells us if the two fractions are equal or which one is greater. This is especially useful when you are working with larger fractions that you aren’t sure how to reduce.
Let’s take a look at some numerical examples.
Find which of the two fractions is greatest.
So, when we cross multiply we get 128, and 182. So, we know that the 7/32 is greater than 4/26 because 182 is greater than 128.
We must always remember that the number that we multiplied with our numerator always represents the fraction with that numerator. I mention this, because it may be a little confusing to see numbers taken from two different fractions being multiplied together, but that product only representing one fraction and not the other. Well, the number with a numerator in it is representing the fraction that that numerator is in. Like, in this example: 4×32=128. 128 goes on the left side to represent 4/26 and 7×32=182 represents 7/32.
Cross multiplying fractions helps us to see if numbers are equal, and if not which is bigger and which is smaller… But that is not it’s only use. Cross multiplying fractions can help us to solve for unknown variable in fractions.
Let’s say we have a fraction 9/16 = x/27. We can cross multiply anytime we have a fraction that is set equal to another fraction. Now, to cross multiply we do the exact same thing that we did in our last example. We take the numerator of one side and multiply it times the denominator of the other side, and we do this same the from the numerator from the other side. In this case, we multiply 9×27 to get 243, then we multiply x*16 to get 16x. So, we have 243=16x. Now, all we have to do to get x by itself is divide both sides by 16. Which, gives us x=243/16. We do this exact same thing even if x is in the denominator.
I hope that this video over cross multiplying fractions has been helpful to you. If it was helpful, and you would like further help you can subscribe to our channel by clicking below. See you guys next time!
Provided by: Mometrix Test Preparation
Last updated: 05/30/2018