SAT Math Question?

<p>Paul drives from his apartment to his parents' house and back. On the trip to his parents' house, he travels at an average speed of 60 miles per hour. On the return trip, Paul drives at an average speed of 80 miles per hour. Which of the following is the closest approximation of Paul's average speed, in miles per hour, for the round trip?</p>

<p>A. 60
B. 68.6
C. 70
D. 71.4
E. 80</p>

<p>No idea what the answer is, and the book doesn't tell me either. It also says "Don't pick obvious answers (obvious answer being C).</p>

<p>I believe that the answer is B. 2x60x80/140=68.571</p>

<p>Make up numbers and solve using d=rt.</p>

<p>Seachai, thats wrong, but a common mistake on this type of problem.</p>

<p>Sorry, I erased my comments. But I saw average so I thought it is what they were looking for, but if closest approximation, it might be B.</p>

<p>It is B, but I don't know what the **** Linger did. I made up a random distance that is a common multiple of both 60 and 80. I chose 4800.
The equation I used was d=rt. I set d=4800, and r=60. Solve for t and I got 80. Then I set d=4800 and r=80. Solve for t and the answer is 60. Thus, the total time is 140. Now I found distance. Since I used 4800 for d and this problem described the trip as a round trip, the total distance is 9600. Average speed is total distance/total time. In this case, the total distance is 9600 and the total time is 140. Thus the answer is 68.57, or B.</p>

<p>Just used a formula to find the average distance (its very helpful and I advise you memorize it):</p>

<p>2x(rate1)x(rate2)/(rate1)+(rate2)</p>

<p>Holy **** does that formula work all the time? Can it be applied to work problems?</p>

<p>Yes the formula will always work for these type of problems. Not sure what you mean by work problems.</p>

<p>Boy works x amount of hours. Girl works y amount of hours. If they work together on a certain project, how long will it take for the project to be completed?</p>

<p>I've never seen this type of question on the SAT. You will never have to use that formula. However, you may need to use the concept behind the question (a word problem dealing with proportion), which is so elegantly compromised when one relies only on a formula.</p>

<p>I've actually seen this type of question on a past QAS. Of course you don't need to rely on the formula but it's always helpful if you can get the answer quickly.</p>

<p>(1/60)+(1/80)=(2/x)</p>

<p>Solve for x.</p>