The multiples of 15 are 15, 30, 45, 60, 75 and so on. For them to be common, they need to be compared to another set of multiples.
30 and all its multiples
All multiples of 90.
60 and all its 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.
Common denominators of 8 and 15 are 120 and all its multiples.
Multiples of 15 include 15, 30 and 45. For them to be common, they need to be compared to another set of multiples.
1, 3, 5 and 15
They have to be multiples of 15 so all of them will be multiples of 3. They all have to be even so you are left with 30, 60, 90
The common multiples of 15 and 25 are all multiples of their LCM, which is 75. So, the common multiples are 75, 150, 225, 300, 375, 450, 525, 600, and so on.
They are the six multiples of 15.
They are all multiples.
Two common multiples of 21 and 15 are 105 and 315.