D04 Describing physical appearance #1855

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Dec 5, 2018 21:08 Diplomat_Zone_D
In a section of "main language" there is a sentence that I don't understand the grammatical thing. That sentence is:

- He's in his early twenties, quite tall and of about average build with fairly short hair.

In this sentence, there is a word "of" before the word "average". Why the word "of" is written before the word "about"? Is the word "of" necessary?

Thank you in advance for explaining the reason why the word "of" is written.