Need a formula that calculates vacation accrual and usage based on hours worked

Good afternoon. I am new with Excel and I desperately need some help. I was hoping someone could help me.

I need to keep track of employee vacations and I am very new to Excel.

I need a formula to calculate that beginning with their start date, for every 40 hours worked they earn a certain amount of vacation days, and then I need to subtract vacation days already used.

for example: Joe who is salaried, started July 14, 2014 he gets 15 days a year but they are built up during the year. He works a 40 hours per week. So he basically earns 10 hours a month or 1.25 days a month and he has used 4 days already.
But Susie who is hourly with a start date of January 5, 2015, gets 8 hours for every 200 hours worked and the number of hours she works varies from week to week.

Is there any chance someone can help me??

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