<p>Diana ran a race of 700 meters in two laps of equal distance. Her average speeds for the first and second laps were 7 meters per second and 5 meters per second respectively. What was her average speed for the entire race, in meters per second?</p>
<p>each lap is 350 m</p>
<p>350/7 = 50 seconds</p>
<p>350/5 = 70 seconds</p>
<p>700/(50+70)=700/120=5.87 m/s</p>
<p>The trick is that she spends more time running at 5 m/s than at 7m/s, so the answer should be a little less than 6.</p>
<p>I think it's supposed to be 5.8333 m/s, not 5.87 m/s. 700/120 = 5.833 m/s, right?</p>
<p>yup, it's 5.8333. thanks, i understand it now</p>
<p>woops, my bad.</p>
<p>Solution: See formula ....
(2<em>5</em>7)/5+7 = 70/12 = 35/6 </p>
<p>Tip for SAT: If this question shows up as a student provided response (grid-in), you SHOULD try to leave it in FRACTION form. Just see how easy it was to find the wrong 5.87 or enter the wrong number of digits. After all, is it 5.83 or 5.8 or 5.84? It is easy to make a mistake filling the form in. </p>
<p>So the easier way is to enter 35/6. On the SAT, you NEVER need to reduce fractions (as long as you can grid them in)</p>
<p>Tip 2: If this question is a MC, check the provided answers: </p>
<p>A. should be speed 1 or 5
B. Answer X
C. should be trick answer with the straight average of 6
D. Answer Y
E. Should be speed 2 or 7</p>
<p>For this problem, you do not even need to calculate anything. The answer is always between lower speed and the straight average. Pick B and you're done.</p>
<p>Ya, POE works really well with math problems.</p>
<p>twinkletoes -
it helps if you scan relevant posts every day (p.412 #18):
<a href="http://talk.collegeconfidential.com/showthread.php?t=85528%5B/url%5D">http://talk.collegeconfidential.com/showthread.php?t=85528</a></p>