Mem = 40, ordinal 13, reduced 4, final form ם = 600. Spelled in full, מם = 80, or מים = 90.
Meaning
Mem is 'water', one of the three mothers with aleph and shin, and the letter of the sea, the womb and the hidden thing — its open and closed forms are read as revealed and concealed water. Its 40 is the number of the years in the wilderness, of the days Moses spent on the mountain and of the days and nights of the rain in the flood, so the letter's number is the number of the tradition's great forty-day spans.