Three night security guards, Robert, Sheila and Tom, are assigned to guard a factory for five consecutive nights, Monday night to Friday night inclusive. Two of the guards are to be on duty each night. Tom must be on duty for four nights and Robert and Sheila are on duty for three nights each. Regulations do not allow any guard to work for more than two consecutive nights. If Robert is on duty on Friday night, then the nights when Sheila is on duty are