Final answer:
In constructing question tags for imperatives, 'Please come in' should be followed by 'won't you?' and 'Let's walk a bit faster' by 'shall we?'. These tags are used to request confirmation or agreement from the listener.
Step-by-step explanation:
A question tag is a short question at the end of a statement that transforms the statement into a question. Looking at the sentences provided, the correct question tags should refer back to the imperatives 'come in' and 'let's walk', which are tricky since imperatives don't have a proper subject. However, for the purposes of question tags, we can treat 'please come in' implicitly as 'will you' and 'Let's walk a bit faster' as 'shall we'. Therefore, the question tags would be:
- Please come in, won't you?
- Let's walk a bit faster, shall we?