Andy walked 3/4 of a mile to the post office and another 1/2 to the supermarket. What is a common denominator for 3/4 and 1/2? 10,8,6,5

A common denominator is very similar to the idea of a common multiple. You can think of a common multiple between two numbers as some number that they both go into. In other words, if you started counting by 3s and 5s, 15 would be a common multiple, since you manage to hit that number in both of your counting lists

3, 6, 9, 12, 15

5, 10, 15

Here, we're looking for some number that's a common multiple of 2 and 4. Counting by both numbers and listing those out:

2, 4, 6, 8

4, 8

We find that 8 fits the bill best. 10 and 6 are both multiples of 2, but not of 4, and 5, being an odd number, isn't a multiple of either one.