To compare these two fractions, we can convert them to have a common denominator.

First, we need to find the least common multiple of 28 and 7, which is 28.

Next, we can rewrite the fractions with the common denominator of 28:

15/28 = 15 * 1/28 * 1 = 15/28

5/7 = 5 * 4/7 * 4 = 20/28

Now we can see that 15/28 is less than 20/28.

Therefore, 15/28 is less than 5/7.