The bike that has a marginal cost of $20 is bike 4.
What is marginal cost?
Marginal cost is the change in total cost when output is increased by one unit. Output is maximised when marginal cost is equal to marginal revenue. Tracing $20 to the vertical axis, the marginal cost of producing bike 4 is $20.
For example, when total cost for 2 units of output is 20 and total cost for 3 units is 30. Marginal cost of producing 3 units is 10. (30 - 20) / (3 - 2).
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