This question may sound hard to English learners and too basic for English speakers. So, the placement of the question on the appropriated Stack Exchange website (ELL or English Native Speakers) can be arbitrary. The thing is: the use of the preposition "of" after numbers, amounts and measurement units always confuses me.
What is correct?
-3 kilograms OF oranges or 3 kilograms oranges?
-80% OF humidity or 80% humidity?
-At the spacing OF 3 meters or at the spacing 3 meters?
-100 kg OF N or 100 kg N?
-30 inches OF precipitation or 30 inches precipitation?
-50 meters OF height or 50 meters high?
According to Cambridge Dictionary preposition "of" is:
used after words or phrases expressing amount, number, or a particular unit
BUT, at the same time, I have found many scientific papers and websites not using the preposition "of" after such numerals. The Cambridge Dictionary does not say anything about cases of non-utilization of the preposition "of", when it is used and when it is okay.
Please, inform which of the pairs are correct. Especially if there is a general rule or common usage, you don't need to specify the correctness of each sentence. Just tell me if using "of" after the numerals/units is correct.