From 2nd customer for each customer the purity of oil becomes 0.9 of it's previous value i.e. 2nd customer will get only 40×0.9=36 liters of pure petrol 3rd customer will get 36×0.9=32.4 liters of pure petrol and so on. Therefore the 7th customer will get 40×(0.9)6=21.25 liters of pure petrol Therefore our answer is option 'B'.