How to convert a cell with a duration (datetime difference) to number in Numbers

I have four fields in Numbers on macOS:

1. F1: datetime
2. F2: datetime
3. F3: duration
4. F4: number

The dateime-fieds contain only a time, like:

10:00    11:00
11:00    12:30
13:00    13:15

The duration-field is calculated by this formular:

F2 - F1

And shows something like:

1h
1h 30m
15m

This works fine.

Now I want to calculate F4 which is a number representation of the duration-field F3. For the examples above it should result in:

1
1.5
0.25

Unfortuanteley I can't find a way to achieve this. How can this be solved?

What I've tried so far:

• changing the fomatting to number didn't work as it will always reset to duration
• setting the field F3 to seconds-only and then calculating F3 / 3600, but the result will always be a new duration