Hello there.

There are many common denominators that you can use. The easiest way to find a common denominator is to multiply the denominators together. In your question, that would be 5 x 7 x 8 = 280. That's not the
lowest common denominator though. This question is likely trying to show you that you should reduce your fractions as much as possible before trying to find a common denominator. What is 4/8 equal to? it is 1/2. So replace it and then try to find a common denominator of 3/5, 2/7 and 1/2. Then you don't have to deal with such large numbers as 280.

Hope this helps,
Stephen La Rocque.