Rowan has been spending four nights studying for her economics tests and her current grade in the course is 75 percent. She decides to take a part-time job and study only three efights for her next test. As a result, after her next test, her grade falls to 60 percent. What is Rowan's marginal cost of working one night a week? Rowan's marginal cost of working one night a week is __ A. 15 percentage points B. 75 percentage points C. 60 percentage points D. greater than 60 percentage points and less than 75 percentage points

## Step 1<br />The marginal cost is the cost of the next best alternative foregone. In this case, Rowan's marginal cost of working one night a week is the difference in her grades before and after she decided to work.<br /><br />## Step 2<br />Before she decided to work, Rowan was studying four nights a week and her grade was 75%. After she decided to work, she was studying three nights a week and her grade fell to 60%.<br /><br />## Step 3<br />To find the marginal cost, we subtract the final grade from the initial grade. <br /><br />### **Marginal Cost = Initial Grade - Final Grade**<br /><br />## Step 4<br />Substituting the given values into the formula, we get:<br /><br />### **Marginal Cost = 75% - 60% = 15%**