Dozen/kilogram or Dozens/kilograms

1. I bought two dozen of mangoes.
2. I bought two dozens of mangoes.
3. My office buys five kilogram of paper every month.
4. My office buys five kilograms of paper every month.

In 1& 2 I think 1 is correct. And hence in 3&4 I think 3 should be the correct one.

It is my exam question and in provisional answer key suggest 1 and 4 is correct. Please explain why it is so.

1 Answer

In American English neither 1 nor 2 is correct. (2 could technically be correct, but is hopelessly archaic, and most native speakers wouldn't even recognize it as right anymore.) The right way to say this is "two dozen mangoes", without "of". This is because "dozen" is just part of the number, and just as you wouldn't say "I bought two of mangoes", you wouldn't say "I bought two dozen of mangoes."

On the other hand, 4 is correct because "kilogram" isn't part of the number, it's the unit that the number is counting in. In this case, whether you're buying a weight of paper (non-count noun) or mangoes (count noun) or sheep (count noun with the same plural as its singular), you'd say "kilograms of".