Absence Accrual based on hours worked for a week in fast formula

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We have a simple requirement. If a person works 20 hrs a week then he will accrue 0.5 hours of sick leave and if person works 40 then 1 hrs and if 60 then 1.5 hrs of accrual.

Now I am not able to find exact database item name to find out hours worked by a person in a week. 

please let me know database item name or how we can proceed with designing this accrual based on hours worked for a week.

Any help would be much appreciated.

Regards