BTGmoderatorDC wrote:


The figure above represents a network of one-way streets. The arrows indicate the direction of traffic flow, and the numbers indicate the amount of traffic flow into or out of each of the four intersections during a certain hour. During that hour, what was the amount of traffic flow along the street from R to S if the total amount of traffic flow into P was 1,200 ? (Assume that none of the traffic originates or terminates in the network.)

A. 200
B. 250
C. 300
D. 350
E. 400


OA B

Source: Official Guide

First let's look at intersection P. Since 1,200 cars flow into P and 900 flow out of P northwardly, 300 must flow out of P eastwardly to Q.

Next, let's look at intersection Q. Since 300 + 500 = 800 cars flow into Q and 200 flow out of Q eastwardly, 600 must flow out of Q southwardly to R.

Lastly, let's look at intersection R. Since 600 + 400 = 1,000 cars flow into R and 750 flow out of R southwardly, 250 must flow out of R westwardly to S.

Alternate Solution:

The total traffic flow into P is the sum of the traffic flow from S to P and the traffic flow into P from east, which is 800. Since the total traffic flow into P is given to be 1,200; the traffic flow from S to P must be 1200 - 800 = 400.

We see that the total traffic flow out of S is 400 + 400 = 800. Since the traffic flow into S from south is 550, the traffic between R and S must be 800 - 550 = 250.

Answer: B