Remember that Marginal Cost (MC) is just the change in total cost due to a change in output. We've added a MC to the table to the right, but we don't need it to answer the question. If we examine the Total Cost column to see how much cost increased when April Showers increased production from 3 to 4 umbrellas we see that cost were 39 at 3 umbrellas and 60 at an output of 4. So MC for the 4^th umbrella is 60 - 39 = 21.