Question 508023
First get their common divisor:

To do that, prime factorize each one, that is re write the number as a product of primes:
 28 = 2*2*7
 40 = 2*2*2*5
 56 = 2*2*2*7

Next, from the lists of prime factorizations find the common numbers. In this case, we only have 2*2 which is common to all lists. So the greatest common divisor in this case is 2*2 which is 4.

Now, to find the least common multiple, multiply the numbers in the given question and then divide it by it's greatest common divisor. In this case, (28*40*56)/4 = 15,680