How SAP calculate monthly working hrs in infotype 7?

Hi, all,

Can anyone tell me how SAP calculate the monthly working hrs in infotype 7?

My work schedule rule:

PWS Week 01 02 03 04 05 06 07

DUMY 001 DUM DUM......

(where DUM with 8 planned working hours)

Employment percent: 100

Daily working hours: 8

Weekly working hours: 40

Monthly working hrs: 171 <- how it calculated?

Annual working hours: 2086 <- how it calculated?

Weekly workdays: 5

Best regards,

Anne