mass transfer

Component A in a feed mixture containing 60%B and 40%A by mass is extracted with pure Solvent C in a countercurrent extraction system. The feed/solvent ratio for this system is set to 3 . Determine the theoretical number of shelves required to obtain 10% by mass of component A in raffinate as a result of extraction. a) 8 raf b) 4 raf c) 6 raf d) 7 raf e) 2 raf