By means of a plate column, acetone is absorbed from its mixture with air from a "nonvolatile" absorption oil. The entering gas stream contains 30 mol% acetone while the entering oil stream is acetone free. Of the acetone in the air stream, 97% is to be absorbed and the concentrated liquor at the bottom is to contain 10 mol% acetone. The equilibrium relationship for this system is y = 1.9x. Determine the number of ideal stages.