What is the right calculation to report the standard on Service Availability?

The formula to calculate Service Availability is:

Total Number of Public Payphone in proper working condition (A) ÷ Total Number of Public Payphone (B)

(A) End of the month data of number of Public Payphone in proper working condition, for the whole 6 reporting months

(B) End of the month data of total number of Public Payphone, for the whole 6 reporting months