My problem was with the MCQ first linear programming question (shadow price). I put the optimal x (1000 units) and y (1800 units) in the labour constraint equation and got the answer but putting it in the material constraint equation, I got 24,000 whereas it was supposed to give me 27000. Then I thought there was a problem with the question.
I chose 75 since the question asked for Relevant cost of both labour and variable overhead costs. That is Labour + Var. overhead+ opportunity cost for labour.