<p>question about Xiggi's rate formula...i just want to confirm</p>
<p>2XR1xR2/(R1+R2) gets you the distance for the round trip in one hour?</p>
<p>Lets say a car travels 45 mph from home to work, and on the way back from work to home he travels 30 mph. Trip both ways in total took 1 hour. How many miles is it on the way there (home to work)?</p>
<p>So you do 2x30x45/(45+30) = 36
but you divide it by 2 since they are only asking 1 direction? so 18</p>
<p>what if the total there and back took 2 hours?</p>
<p>can someone explain how to use the formula in different cases or how it is derived? </p>
<p>I'm not up-to-date on the online version, but a couple of years ago the advantage was that they offered extra practice tests that weren't in the Blue Book. If that's the case and you will exhaust your supply of CB practice tests it might be worth it, but also make sure that they also supply the solutions (not just the ABCD answers) to the problems.</p>
<p>question about Xiggi's rate formula...i just want to confirm</p>
<p>2XR1xR2/(R1+R2) gets you the distance for the round trip in one hour?</p>
<p>Lets say a car travels 45 mph from home to work, and on the way back from work to home he travels 30 mph. Trip both ways in total took 1 hour. How many miles is it on the way there (home to work)?</p>
<p>So you do 2x30x45/(45+30) = 36
but you divide it by 2 since they are only asking 1 direction? so 18</p>
<p>what if the total there and back took 2 hours?</p>
<p>can someone explain how to use the formula in different cases or how it is derived?</p>
<p>This is actually a formula for the average speed on a two-leg trip with EQUAL legs.</p>
<p>Assume each leg is D, then total travel distance is 2D.
Total travel time
D/R1+D/R2.
Average rate
2D / (D/R1+D/R2) =
2D / (D(1/R1+1/R2)) =
2 / ((R2+R1)/(R1xR2)) =
2xR1xR2/(R1+R2) mi/hr.</p>
<p>If travel time is 1 hour, then total trip is
2xR1xR2/(R1+R2) x 1hr =
2xR1xR2/(R1+R2) mi.
For a round trip you divide it by 2 to find one leg.
Substitute 1hr with any other time in the above calculations to answer a modified question.</p>
<p>2 things i want to clear up. For a round trip why would you divide by 2? Wouldn't the formula as is give you the round trip? I thought you divide by 2 to find the distance of one direction. Sorry I think i am misunderstanding that "For a round trip you divide it by 2 to find one leg." statement</p>
<p>and for example, if they say the travel time is 3 hours, and they wanted you to find the distance of one way, would you do R1xR2/(R1+R2) x 3 hours</p>
<p>^^ diligency, when I say "For a round trip you divide it by 2 to find one leg" I mean the same thing as you do when you say "you divide by 2 to find the distance of one direction". I consider a round trip consist of two equal legs - forth and back.</p>
<p>Your example with 3 hours is correct. I would still prefer memorizing and dealing with the formula for an average speed 2R1R2/(R1+R2).
There was a question on the January 2008 SAT about a car which traveled two given equal legs; it asked what was the average speed for the whole trip.
With the above formula you don't even need to know the distances:
look, Ma, no legs! lol</p>
<p>I doubt a question on the average speed will ever appear on the SAT in this form (too simple), but just in case:
if Leg1 =/= Leg2, but
T1 = T2,
then the average speed =
<a href="R1+R2">b</a>/2**.</p>
<p>For example, if a car traveled 20 miles at 30 mi/hr and then 40 miles at 60 mi/hr, then the average speed for the trip was
(30+60)/2 = 45 mi/hr,
because
T1 = 20/30 = 2/3 hour, and
T2 = 40/60 = 2/3 hour too.</p>
<p>To verify:
the total travel time is
2/3 + 2/3 = 4/3 hr,
or, another way,
(20+40)/45 = 4/3 hr.</p>
<hr>
<p>By contrast,
if a car traveled 20 miles at 30 mi/hr and then 20 miles at 60 mi/hr, then the average speed for the trip was
2(30)(60)/(30+60) = 40 mi/hr,
because
Leg1 = Leg2 = 20 miles.</p>
<p>To verify:
T1 = 20/30 = 2/3 hr, and
T2 = 20/60 = 1/3 hr,
the total travel time is
2/3 + 1/3 = 1 hr,
or, another way,
(20+20)/40 = 1 hr.</p>
<hr>
<p>Finally, one more unlikely version (just in case, again):</p>
<p>if a car traveled 30 miles at 40 mi/hr and then 25 miles at 50 mi/hr, then the average speed for the trip was
(30+25)/(30/40+25/50) = 44 mi/hr.</p>
<p>So I just took my first test (from the BB) using the Xiggi method (only used dictionary for reference, and did sentence completion q's a few days ago) but I did sort of follow the time limits (took the whole test w/o essay under 4 hrs, didn't time any specific section) and I got 2040 (took lowest scores from each section). Is this good or bad?</p>
<p>The BB is MUCH easier than the Barron's book, which I have completed 5 tests from. But this test seemed suspiciously easy .. and I know CB is capable of fooling students because their practice PSAT exam was ridiculously easy (I got 220+) while on the real PSAT I got a 194.</p>
<p>okay, new to the boards 3 1/2 years later. Xiggi, did you ever take your tips to a publisher? before I go through and print out what you wrote, I'd go buy your guide if it exists!</p>
<p>Ren, I'm trying to figure this stuff out. In your post (#727), you mentioned BB/online and QA's. Also in this post CB's. Could you tell what those are. Thanks, officer</p>