Jawabanmu

2014-03-06T20:28:54+07:00
D = fob + fok <--> fok = d - fob .......... (1)
M = fob/fok = 20fok = fob ..................(2)
fok = d - 20fok <--> 21fok = 4
fok = 4/21

Semoga benar ya :)