How many days will the woman be traveling?

W: When do you leave for holiday?
M: On the twentieth. We’re flying. The flight takes seven and a half hours.
W: And when are you back?
M：On the twenty-eighth. I will give you a ring when we’re back.

W: Can you play tennis with me today, Bob?
M: I am afraid I can’t. I’ll have to go and help Emily with English. If she fails in the final examination, she won’t be able to go on the trip with us.
W: I see. Good luck to her.
M: Thanks.