If I understand you correctly, the "standard" work week is 6 days of 7.5 hours each, or 45 hours, and the "alternative" week is 5 days of 7.5 hours, or 37.5 hours.
Therefore,I would set the Periodic Work Schedule with
Weekly Working Hours = 45.00 hours (6 x 7.5 h),
Ànnual Working Hours = 2340 hours (45 hrs/wk x 52 weeks), and
Monthly Working Hours = 195 hours (2340 hrs/yr / 12 mo)
Alternatively, you could set-up the Weekly Working Hours as the average of (Wk1+Wk2)/2, or
Weekly Working Hours = 41.25 hours (sum of (45+37.5)/2),
Ànnual Working Hours = 2145 hours (41.25 hrs/wk x 52 weeks), and
Monthly Working Hours = 178.75 hours (2145 hrs/yr / 12 mo)
but you would have to be aware of how you calculated these numbers when using these values in a pcr (as mentioned by Junga Gunja).