What was the question for this implicit rationale, and corresponding answer?
Beth had 150 students her 10th - grade year, and added 30 more the following for a total of 150 + 30 = 180 students. In her final year, 15 students left her class, bringing her total to 180 - 15 = 165 students.
 The answer: 165
The question: Beth had 150 students in her 10th-grade class. The following year 30 more students join. In her final year, 15 students left. How many students did Beth have in her class at the end of her final year?