Student or Learner
I've been learning compound adjectives recently and I can't figure it out when to add another adjective-component to the "numeral+noun" phrase which is moulded into a compound adjective. Why is it 'a two-year-OLD boy', but 'a two-hour talk'? How can we make sure where to add extra-adjectives (like 'old' in the example given) and where to make do without them? Why will it be 'a two-kilo bag', and not 'a two-kilo-heavy bag'? Why is it not "a two-metre gorilla", but "a two-metre-tall gorilla"? Do we write 'two-page compositions' or 'two-page-long compositions'? What's the rule for such cases or at least the rule of thumb or logic??? I've tried to find the answer to this question in my books, but failed. I really hope that I'll get help here.
Thank you very much for your help!
There's no rule—you just have to get used to the colloquial usage.
'A two-year-old boy', 'a two-metre-tall gorilla', 'a 400-page-long book' are all OK.