The LCM of (9,15) is 3.
9 = 3 * 3
15 = 5 * 3
therefore
3 is a common multiple. also three is the lowest multiple.
therefore...
LCM (3,15) = 3
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The common multiples of 9, 15, and 20 are an infinite set of multiples of their least common multiple, which is 180. so the common multiples of 9, 15, and 20 are 180, 360, 540, 720, 900, 1080, 1260, 1440, 1620, 1800, and so on.
Multiples of 3 include 3, 6, 9, 12, 15, and so on. To be common, they need to be compared to another set of multiples.
There are infinite common multiples for a set of numbers. So it is not possible to write down all the common multiples. Some common multiples of 15, 30 and 45 are 90, 180, 270, 360,... There is another way to find common multiples: Write the multiples of LCM(15, 30, 45). Actually the common multiples of a set of numbers are the multiples of their LCM. LCM of 15, 30 and 45 is 90. And the multiples of 90 are 90, 180, 270, 360, 450, 540,... which are the common multiples of 15, 30 and 45.
3:3,6,9,12,15,18,21,24,27,30,33,36,39,42,45,48,51,54,57,60,63,66,69,72,75,78,81,84,87,90,93,96,99 15:15,30,45,60,75,90,105
The common multiples of 9 and 11 are 99, 198, 297 and so on.