## albertw 3 years ago can someone explain to me how to add 5/20 and 6/15 I don't understand lowest common denominators?

1. misterf

A really easy way to do this (although not the most efficient) is to just start listing all the multiples of the denominators:

2. julie

Yeah, basically you can't add two fractions unless they have the same denominator.

3. misterf

the denoms are 20 and 15, so list the multiples: 15: 15, 30, 45, 60, 75 ... 20: 20, 40, 60, 80 ...

4. misterf

here you can move from left to right until you find one that's in both lists :)

5. albertw

hmmm, ok, so how would I add them?

6. julie

misterf nailed it :)

7. julie

Ok so once you find the common denominator -- above, we see that it would be 60.

8. siddharth

Well, to add or subtract fractions, you have to make sure the bottom numbers on both fractions are the same. So, the best way to add two fractions with different denominators is to find the Lowest Common Multiple, and change the fractions to that form to perform the addition.

9. julie

Then you find how you make the denominators of both the same as that common one. So 15 * 4 = 60 and 20 * 3 = 60.

10. julie

Then you multiply the numerator, too: 6 * 4 = 24 15 * 4 = 60 and 5 * 3 = 15 20 * 3 = 60

11. misterf

$\frac{5}{20} = \frac{15}{60}$ $\frac{6}{15} = \frac{24}{60}$ then add them normally: $\frac{15}{60} + \frac{24}{60} = \frac{39}{60}$ does this make sense?

12. julie

Haha misterf beat me to it :)

13. misterf

please gimme a medal if i helped i am collecting them :p

14. julie

When you add two fractions and they have the same denominator, the denominator stays the same and you just add the numerators.

15. julie

Gave it! :)

16. siddharth

Misterf explained it really nicely. Let us know if you don't understand something, albertw. I gave you a medal as well, misterf :)

17. misterf

yay!!