Hence the length of a lunar month is either 29 days (40 percent of the time) or 30 days (60 percent of the time), and no other length.
(Hence the length of a lunar month; is; either 29 days (40 percent of the time) or 30 days)
Hence the length of a lunar month
is
either 29 days (40 percent of the time) or 30 days