15 hours-what type of noun

Answer 1

The term '15 hours' (fifteen hours) is a noun phrase, a group of words based on a noun (hours) that functions as a unit as a noun in a sentence.

The noun '15' (fifteen) is functioning as an attributive noun (a noun functioning as an adjective) to describe the noun 'hours'.

Examples:

Fifteen hours is the estimated driving time. (subject of the sentence)

We drove for fifteen hours to get here. (object of the preposition 'for')