## Elementary Algebra

First, we express each number as a product of prime factors: $20=2\times2\times5$ $28=2\times2\times7$ The least common multiple includes each different prime factor as many times as the most times it appears in one of the factorizations. The most times 2 appears is twice. The most times 5 appears is once. The most times 7 appears is once. Therefore, LCM$=2\times2\times5\times7=140$