[Forms] How could I calculate the time from 2 textboxes?

I'm doing the overtime form on Microsoft forms, I have 4 questions:

1. I use the text-box for time start and end like the image below, how could it calculate the total hour instead of number '10.'?

2. From the image above in the section 2 of form is the first time OT of the month, second time OT require another section/page like this. Could the form ask like 'Do you have another OT?'? If 'Yes' it will next to page 3 with same page form, however, if 'No' it will finish the form.

3. How could auto run the document number?

4. How could link the type of OT and total hour? I would like to know which type is the most number of hours.

Thank you very much.