At 20 bikes, the average cost is $200. This means that each bike costs about $200 to make, thus the total cost for the first 20 bikes is $4000.
The marginal cost represents the cost of producing one more unit. If the cost of producing a bike after the 20th one (in other words the 21st bike) is $150, then the total cost of producing all 21 bikes is $4150.
