The marginal cost is how much it costs to produce one item more. It cost 0$ to produce 0 bikes but if you make one, then it costs 80$. Hence the marginal cost for the first bike is 80$. For the fourth bike, we have that the 3 bikes cost 110$ and the 4 bikes cost 130$. Hence it costs 20$ dollars to produce one more bike (from 3rd to 4th) and hence the marginal cost is 20$. Similarly for the 6th bike, we compare the cost of producing 5 bikes to the cost of producing 6 bikes and we get that the marginal cost is 50$. Applying the same methodology, the marginal cost for the 7th bike is 60$.
