A number is an exact multiple of each of a group of numbers. For example, 15 and 30 are common multiple of 3 and 5.
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When one of the numbers is a multiple of the other.
There is no "greatest common multiple" of any two numbers. Whatever their product is, it can be multiplied by any positive integer to yield an even greater number that is also a multiple of the first two. Thus, the number of multiples is infinite.
The LCM of 15 and 25 is 75. The least common multiple of two numbers is the product of the two numbers divided by their greatest common factor. The greatest common factor of 15 and 25 is 5. (See related question "What is the greatest common factor of 15 and 25?" for this calculation.) So, the least common multiple is 15 x 25 ÷ 5 = 75. The least common multiple (LCM) is often also called the lowest common multiple or smallest common multiple. Keep in mind that these different terms all refer to the same thing: the smallest positive integer which is a multiple of two or more numbers.
There is an infinite number of common multiples for 3 and 15. A common multiple of any two or more numbers is any number into which each of two or more numbers can be divided evenly (zero remainder). However, the least or lowest common multiple (LCM) of 3 and 15 is 15.
There is none because the Least Common Denominator (or LCD) refers to the Least Common Multiple (LCM) of the denominators of two or more fractions or mixed numbers. Even if you converted your whole numbers (x) to their fraction equivalents (x/1), the LCD would always be 1.