The multiples of any number are simply the product of the number and the natural numbers 1, 2, 3, 4, 5 and so on.
Thus the multiples of 5 are: 5x1, 5x2, 5x3, 5x4, 5x5, 5x6, etc. In other words, 5, 10, 15, 20, 25, 30, and so on.
The multiples of 16 are therefore: 16, 32, 48, 64, 80, and so on.
Multiples of 13, like 13, 26 and 39.
The first 3 multiples of 13 are: 13, 26, and 39.
The multiples of 16 are 16, 32, 48, 64, 80, etc. The multiples of 20 are 20, 40, 60, 80, 100, etc. The common multiples of 16 and 20 are 80, 160, 240 and so on.
16, 32, 48. Since 16 is divisible by 4. Every multiple of 16 is also a multiple of 4, so I just took the first 3 multiples of 16.
The common multiples of 6 and 13 are the infinite set of multiples of their LCM, which is 78. So the common multiples of 6 and 13 are 78, 156, 234, 312, 390, and so on.