The Tan family of seven own a shop called 'Tan & Tan Company'. Exactly one of the members is on duty everyday. Find out who is on duty on Sunday given the following:
Mr Tan is on duty on a weekend.
The two females are on duty on consecutive days, with one of them working in the middle of the week (Sunday is considered the start of the week).
Billy is on duty the day just before Mr Tan.
Billy's three brothers are on duty on consecutive days. The youngest of the three is on duty the first day, followed by the one in the middle, then the oldest.
Give your answer as a number based on DECREASING order of age: