Boy: And 6 ______ don't we revise together sometimes?

Explanation:

The dialogue is about exam preparation. The girl has just agreed that taking notes is a good idea. The boy then proposes another study method: revising together. The phrase 'And 6 ______ don't we revise together sometimes?' is a suggestion phrased as a question. The word 'why' fits perfectly to form the question 'And why don't we revise together sometimes?', seeking agreement or a reason to do so.

Answer:

Boy: And 6 why don't we revise together sometimes?