We have come to a question about a 6 foot long creature (FCE Star, p.76 l. 23). Typically you would say 6 feet, but here this is also correct, and actually more often used in this situation.
The rule could go like that: if we can see an adjective in the meaning (a four-year old boy - čtyřletý chlapec), we use the singular and we can also use the hyphen (-), whreas when it is not an adjective (the boy is three years old), we use the plural and no hyphen. Thus, we have a six foot long monster (šestimetrová) but a monster that is six feet long ()dlouhá 6 metrů.
Let's see what information we learn about that from good English information sources:
The ordinary plural of the unit of measurement is feet, but in many contexts, foot itself may be used (a six-foot tall man). This is a reflex of the Anglo-Saxon (Old English) genitive plural.(Wikitionary)
Note that we would normally say six foot despite the plural reference, although six feet is also possible.(BBC Learning English)
In Standard English, foot and feet have their own rules when they are used in combination with numbers to form expressions for units of measure: a four-foot plank, but not a four feet plank; also correct is a plank four feet long (or, less frequently, four foot long). When foot is combined with numbers greater than one to refer to simple distance, however, only the plural feet is used: a ledge 20 feet (not foot) away. At that speed, a car moves 88 feet (not foot) in a second.
Some people in New England and the South use constructions such as three foot and five mile in place of Standard English three feet and five miles in certain contexts. Some speakers extend this practice to measures of time, as in He was gone three year, though this is not as common. Interestingly, such constructions are used only if a specific numeral (other than one) precedes the noun. Thus, She gave me four gallon of cider can be heard in vernacular speech; however, no one would say She gave me gallon of cider for She gave me gallons of cider. This is because the numeral makes apparent the plural meaning that would not be specified if both the numeral and the plural form were omitted.
We get a reference to the
Why do we say "30 years old", but "a 30-year-old man"?
This pattern goes all the way back to Old English (alias Anglo-Saxon). It's the same reason many of us say that someone is "5 foot 2" rather than "5 feet 2".
The source of the idiom is the old genitive plural, which did not end in -s, and did not contain a high front vowel to trigger umlaut ("foot" vs "feet"). When the ending was lost because of regular phonetic developments, the pattern remained the same, and it now seemed that the singular rather than the plural was in use.