<p>Can somebody explain why it works / how he came up with it? I dont want to use it without knowing why it works =</p>
<p>Xiggi's formula can be found here:</p>
<p>its post #193</p>
<p>Do you mean d=rt? he didn't come up with that it is just a basic formula. You could explain why it works with calculus but it is easier to just think about it like this. If you drive at 50mph(that is the rate) for 2 hours(time) how far will you go? You can easily see that the formula works. Also you can rearange the formula like xiggi did to solve for variables other than d. Just trust that it works and it can help you out.</p>
<p>you can easily get d=rt from dimensional analysis</p>
<p>but i think he's talking about
(2xy)/(x+y)</p>
<p>It's the harmonic mean of x and y and it gives the avg. speed of a person who goes a certain distance at x speed and then the same distance at y speed . And xiggi did not come up with harmonic mean.. it has been around for a long time.</p>
<p>FYI It's not just (x+y)/2 because the person spends more time going the slower speed</p>
<p>LOL, have I ever said I came up with the harmonic mean formula? I merely posted about using a formula that is well beyond the scope of the SAT to solve a SAT problem in the quickest and easiest way. Despite being one of the easiest to solve, this type of problems trips the MAJORITY of the students. Anyone familiar with the formula or the way ETS poses the question in a MC should solve it without breaking a sweat. </p>
<p>
<p>It is EXTREMELY doubtful that ETS will ever go beyond presenting problems with very simple data such as 1 hour roundtrip and very basic rates. There is really no reason to explore the formula for harmonic means beyond its simplest form.</p>
<p>While it is rather easy to show how the formula is derived from the basic d = r*t, you can safely assume it works without having to know its entire origin.</p>
<p>When this type of problem shows up on the SAT, it takes two quick steps:</p>
<ol> <li>Recognize the CORRECT problem and verify the variables</li> <li>Use the quick and dirty formula or know how to pick the answer that ALWAYS works if the questions is a MC.</li> </ol>
<p>Do not make it harder than necessary!
</p>
<p>And here is an ever older quotation:</p>
<p>
[quote]
A girl rides her bicycle to school at an average speed of 8 mph. She returns to her house using the same route at an average speed of 12 mph. If the round trip took 1 hour, how many miles is the round trip.</p>
<p>A. 8
B. 9 3/5
C. 10
D. 11 1/5
E. 12</p>
<p>Apply the formula for the average speed => 2 * 8 * 12 / 8 + 12 or 192/200 or 9.6 or 9 3/5. Since the girl rode her bicycle for ONE hour, she rode a distance of 9.35 miles total. That is all you need. There is no glory whatsover in playing with the d=rt formula. </p>
<p>Also, a small addendum: if this problem shows up in a multiple choice, you do not need to even lift your pencil, except to mark B.</p>
<p>There three excludable answers 8 and 12 for obvious reason. The trap answer (chosen by a majority of students) that is 10. That leaves only 9 3/5 and 11 1/5. Well, the answer is ALWAYS slightly below the straight average of 10. If there is only ONE number between the straight average of 10 and the lowest speed, just mark it and move on.
[/quote]
</p>
<p>As I said, keep it simple!</p>