please help me to understand using "an average of" in this sentence:
Husbands help in the house more than they did in the past, but in the UK for example, men do an average of just six hours a week compared to their wives, who do over eighteen hours.
I don't understand why is it used the preposition of. why not: an average without preposition of. What part of speech is "an average of" in this sentence? Is it a noun with the preposition of, or it's an adverb like "a lot of"
There is another example with an average with preposition of. I agree. All in this correct:
The average of 3, 4 and 8 is 5.'