Which is correct or more likely, a/an or one. If both a/an and one are possible, write them both

Which is correct or more likely, a/an or one? If both a/an and one are possible, write them both:

1.  It weights over                hundred kilos.

2.  I only asked for                pizza - I didn't want three of them.

3.  I wouldn't allow               child of mine to be treated in that way.

4.  It only took us             _ week to drive to Greece.

5.  I've always wanted to own               silver - coloured car.

6.                 sandwich isn't enough. I usually eat four or five

7.  Policies differ from                state to another.

8.  Less than three quarters of                hour later, she was home.

9.  All of the competitors completed the race, with just                exception.

10.  She left home late              morning and hasn't been seen since.

11.  The best way to learn             musical instrument is to find              enthusiastic teacher.

12.  Somewhere in the distance,              bell rang.