help needed

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please help i have no clue where to start and its doing my nut in!!!!

so far i have an objective function

obj = f*28 + a*f*32 + 0.4*a*f*45 - 0.1*a*f*190 + c*0.5*a*f*25 + d*0.5*a*f*35 + b*f*31 + 0.37*b*f*45 - 0.08*b*f*210 + c*0.55*b*f*25 + d*0.55*b*f*35 - 0.2*c*0.5*a*f*210 - 0.4*c*0.5*a*f*175 + 0.4*c*0.5*a*f*45 - 0.3*d*0.5*a*f*175 - 0.4*d*0.5*a*f*190 + 0.3*d*0.5*a*f*45 - 0.2*c*0.55*b*f*210 - 0.4*c*0.55*b*f*175 + 0.4*c*0.55*b*f*45 - 0.3*d*0.55*b*f*175 - 0.4*d*0.55*b*f*190 + 0.3*d*0.55*b*f*45

where f = feedstock in tonnes
a,b,c,d are all percentages

the waste bit just seems to go round in circles so dont know how to put that into the objective function
eg. process c produces 0.4 tonnes of waste which goes into process r which produces 0.4*0.3 tonnes of intermediary which goes back into either process c or d producing yet more waste

Since "waste product cannot be disposed of by the plant" it generates no profit and so has no place in the objective function which should be "total profit".

it costs money to go through process r which all waste has to in order to be disposed so it does need to be in the objective function as i am trying to maximise profit which is revenue - costs