Your question doesn't make a great deal of sense. How long will it take the boys (emphasis on the plural) to run 1/5 mile -- simple: as long as it takes the slower one to run 1/5 of a mile. If both of the boys are going to run 1/5 of a mile, it doesn't matter how long it takes Ben to go that distance, because the overall elapsed time is going to be paced by Noah.
On the other hand, if you meant that the boys are going to run 1/5 mile in tandem, as in a relay race situation, you didn't bother to share what portion of the distance Ben is going to cover and which portion Noah is going to cover. One possibility is that they will split the distance equally, but nothing in your problem statement would allow such an assumption as being more likely than any other division of the labor.
And please don't write back to tell me "that's all it says in my assignment sheet/book/whatever" I don't care about that. If what you wrote is all the information you have, then you need to go and get more information from your instructor or from further reading in your text.
John
My calculator said it, I believe it, that settles it
