One and a half hour/hours

English as a Second Language English Vocabulary, Grammar and Idioms
1

Hi

Could you please shed some light on the given two sentences? Are both of them correct or only one?

1- I will be back in one and a half hour.
2- I will be back in one and a half hours.

Tom

2

Good morning, Tom

Only the second one is correct.
I will be back in one and a half hours.

It would be more common to say:
I will be back in an hour and a half.

Amy