What is the lest common multiple of 14 and 21?

Least common multiple is the smallest number is a multiple of both numbers. First the numbers should be factored into its primes.
14 = 7 X 2
21 = 7 X 3
The least common multiple will be 7 X 2 X 3 = 42

42.

There's a trick to finding this easily. Take the prime factors of both numbers.

14 = 7 × 2
21 = 7 × 3

Now, multiply the unlike numbers.

2 × 3 = 6

Multiply the 6 by one of the like numbers.

6 × 7 = 42.

That's it. Hope this helps!