If plant efficiency means they can only produce 80% of their maximum capacity of 400 units, the equation to solve this would be x = 80% of 400, which 0.8 * 400 = 320 units. At 100% efficiency they would produce 400 units, so they've lost 80 units because of reduced efficiency. If, however, their efficiency of 80% corresponds to the 400 units, then their effective capacity at 100% would be given by x in the expression 0.8x = 400. So x = 400/0.8 = 500. It depends how you read the question.