Idioms

Hi! Number 3 is not an idiom. How come 4 and 5 are idioms? Thanks, Fulvio

1. He's a man of his word. (He can be trusted)

2. It's a piece of cake to pass the driving test. (It is easy to pass the driving test)

3. Do you suppose the children would like an ice cream? (Do you think the children would like an ice cream?)

4. What time does it start? At nine, I suppose.

5. Our plane is supposed to take off at 10 am.