What is the highest common multiple of 6 and 15?

Answer 1

There is really no such thing as a "highest common multiple". Once you find the least common multiple of a set of numbers, you can keep adding the LCM to itself over and over again. Each new number you get will be a common multiple of your set of numbers, but each new number will always be larger than the previous. This means that you can keep adding while the number approaches infinity and you will still never find a highest multiple.

Answer 2

The highest common multiple of any set of integers is infinite.