Why do you say “Fifty miles IS …”?

Fifty miles is a long way to ride a motorbike.

Why use is with miles which looks like a plural noun? Can you explain me what the rule is?

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Nice question! I don't have a technical answer, but I think it's because fifty miles is being treated as a singular unit of distance. "X is a long way to ride..." No matter what X is, it's treated as a single unit. Similarly "Five gallons of milk is more than enough for the birthday party." No matter how many gallons there are, you aren't talking about each individual gallon (or mile). You're talking about the mass of the units as a whole. – WendiKidd Dec 4 '13 at 3:55
I've always thought about it as [A distance of] 50 miles is a long way... where now a distance is clearly singular. – Jim Dec 4 '13 at 4:01
Also note that "24 hours makes a day", but "there are 24 hours in a day". – Damkerng T. Dec 4 '13 at 4:32
I think explaining that measure expressions can be notionally singular is simpler than resorting to ellipsis, but I suppose either way works. Still, I'm skeptical. What would be elided from "Another three eggs is all we need"? You can think of an answer, but the fact that it's non-obvious makes me think it's not the right answer. – snailplane Dec 4 '13 at 6:14
@DamkerngT. Actually I'd go with "24 hours make a day"... Just like we say "Two wrongs don't make a right." – WendiKidd Dec 5 '13 at 21:33

Let us compare two key sentences:

There are ten beers in my fridge.

Ten beers is a lot to drink.

The difference is regarding the beers as individual items, versus a single numeric quantity (where "beers" serves as the unit, just like meters, seconds or kilograms).

Similarly:

The last fifty miles of that route are the most scenic! [Among the fifty miles, individually, there are found the best views and points of interest.]

Fifty miles is a long way. [The quantity fifty is a long way, if the units are miles.]

The is/are applies even if the noun is elided, and only the number is present.

How many beers do we have? Twelve are in the fridge, six more in storage.

One or two beers isn't a lot to drink. But ten is.

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Another interesting pair: "Two apples is enough for me." versus the ungrammatical *"Two apples is on the table." – snailplane Dec 4 '13 at 7:42
@snailboat Yet, you can have "two apples are enough". In the Slovak language it gets even more tricky. Two apples uses the nominative case, and so apples is the plural subject, whereas ten apples must use the genitive case which has the sense "of apples", which makes "ten" the subject, necessarily singular. The change starts from the number five: "štyri jablká sú dosť" (four apples are enough); "päť jabĺk je dosť"(five apples are enough; lit: "five of apples is enough"). It stays consistently rigid though: there is no shifting between a individual vs. set view based on the situation. – Kaz Dec 4 '13 at 18:22

Fifty miles here is considered as the entire journey (all those miles collectively). Something like 24 years is not a small period.

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