Free Chapter: GRE Math Review

The following is an extract from the introductory section, fourth chapter of the GRE Math Review. There are totally five chapters in the GRE Math Review, and each chapter is divided into several sections.

Chapter 4: Word Problem

10. Decision Tree

Decision tree is a kind of graphical representation of decisions involved in the choice of statistical procedures. Here, we use this definition to describe a type of math question. A typical decision tree question will be as following:

In a certain college, 80 percent of the freshmen (first-year students) lived on campus, and 60 percent of those who live on campus live in dormitory. If 1,200 first year students live in campus dormitory, how many first year students were there in this college?

(A) 2,500

(B) 2,400

(C) 2,000

(D) 1,500

(E) 1,200

There are two ways to solve this question. The first one is to express this question in a simple equation. But here, I’d like to introduce a diagramming method, also known as decision tree.

Step 1 Divide the first-year students into two groups: one who live on campus (80%) and the other one who live off campus (20%).

Step 2 Further divide the subgroup living on campus into the one who live in dormitory (60%) and the other one who live in apartment (40%).

Step 3 Since the population for sub-subgroup (who live in campus dormitory) is 1,200, then the subgroup would be .

Step 4 Then the total graduating class would be

Therefore, A is the correct answer.