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# Subtracting decimals: 9.57-8.09

To find the least common denominator of two fractions, we can either list out the multiples of the two denominators until we find a common one, or use prime factorization. Once we've found the least common denominator, we can rewrite the fractions so they both have this denominator, and then add them together. Created by Sal Khan.

## Want to join the conversation?

• So slightly off-topic...

I quit marijuana recently, and as I do these exercises and get further into Khan Academy, I keep wondering if people can do these exercises easily in their head. Give me a pen and paper and these problems are like nothing, because I understand them conceptually, but in my head I can barely keep track of the answers generated at each step.

Just curious if I'm the only one that needs a pen and paper for this simple stuff.
• Some people can do these kinds of things in their heads, but don’t beat yourself up if you can’t. It gets easier with practice and as you find your own shortcuts. :)
• I now how to do it but I can't do it sideways I now it doesn't mater if it is sideways or not
• Do it the way that makes it easier for you...but sometime in your life you will have to learn it sideways
• so when you line up all the numbers will you subtract left to right or the other way around??
• Like any regular subtraction problem, you will subtract right to left.
• Great video and now I really now how to subtract decimals. Thank you?
• do we have to line it up exactly?
(1 vote)
• You cant take a hundredth away from a 10th and vice versa. Think of fractions: you can't subtract 3/5 - 1/3, you need a common denominator. The same thing with decimals. So we subtract hundredths from hundredths, and tenths from tenths. And when you don't have enough hundredths, you borrow 10 hundredths from the tenths (1/10=10/100). So once you line it up, it's the same as any other subtraction problem, just with a decimal in the middle.
• if I go on youtube and search this will this vid come up? Or is it only on the site?
(1 vote)
• did you know that this is on the site and on youtube too
• I don't understand this concept on the video
• Try to do the problems without the decimals first and then try to put them back and do them.
It should still all work the same way. The only difference is that the numbers that are to the right of the decimal are just showing that they are units but they are not enough to create 1 whole unit.