Hi Sam,
Don't panic! Congratulations on getting an interview as apparantly Virgin receive 45,000 applications every year and only 20% of those people are invited for an interview!
You will have no problems with the maths test, if I remember correctly you have 10 questions to answer in 9 minutes! I panicked and rushed through but then had lots of time left over afterwards!
Just some examples...
1) You are flying to LAX, the flight is scheduled to depart at 1815, the flight is 8 hours in duration. What time will you arrive in LAX remembering that the local time is GMT-7.
2) A passenger purchases 3 bottles of vodka at £10.50 and a bar of chocolate at £2.00. He gives you a £50 note, how much change will you need to give him?
I found the one to one very relaxed but had been to cabin crew interviews previously so had an idea of what to expect. They will ask you lots of situational questions..a time when you exceeded a guest's expectations, a time when you dealt with an angry pax etc etc.. My suggestion is to think of some situations the night before so that you won't go blank when they ask you!!!
Good luck, you will be great!
