The least common multiple (LCM) of a set of numbers is the lowest value that all of the numbers can be divided by and have a result of a whole number. In this case, the least common multiple is 21 because 21/7=3 and 21/21=1.
The least common multiple is 21. I got this by going through the multiples of the larger number, with the first one being 21. Becuase 21 times 1 is 21 and 7 times 3 is 21 so 21 is the LCM.