<p>how common is a problem like this: if it takes 15 people 8 hrs to make 100 items, then how many hours does it take 6 people to make half the items?
..yea its copied from a few threads ago</p>
<p>also, how often do u see a problem in which the solutino lies in the formula n1*n2/(n1+n2)...in this type of problem, they typically give u 2 rates and a distance, and they ask u find the overall avg rate or something. If some1 knows of a problem, post it, b/c im lookin for one, but cant find it. that formula mentioned above is wrong, i know, but it goes something like that...
lastly, whats the most difficult type of probability seen on the SAT..b/c i dont have the logic for anything more than basic probability...
sorry for the vagueness, but thanks</p>
<p>But the second one... is this what you mean?</p>
<p>Billy rides his bike to school at a rate of 15 mph, but gets picked up by his mom coming home. Assuming Billy's mom's car travels at a constant rate of 30 mph, what is Billy's average speed going to and from school?</p>
<p>Average speed = distance/time</p>
<p>You can assign any arbitrary distance for the distance to school (if it mattered, they'd give it to you). Easiest is probably 30 miles.</p>
<p>So, it takes two hours to get there and an hour to get back (sucks for him); total of three hours.
The total distance would be 30 * 2 = 60.</p>
<p>So avg speed = 60/3 = 20 mph NOT (15 + 30)/2 = 22.5 mph. You see, it takes him twice as long to get home, so he spends more time at the lower speed.</p>
<p>I take a shortcut, because it's easy for me to recognize these problems... I just do (30 + 15 * 2)/3 = 20.</p>
<p>I hope that's what you were talking about, lol.</p>
<p>thanks 8parks, yea..iin77, thats wat i was talkin bout, but i think thats a simple version that i couldve done haha, im a lil thrown off when they give u missing pieces or when they ask u: if it takes 1.5 times as long to get to B from A, as it did to get to A from B, and A and B are separated by X miles...and they ask u the avg velocity or something</p>
<p>if it takes 15 people 8 hrs to make 100 items, then how many hours does it take 6 people to make half the items?</p>
<p>For those type of problems all you have to remember is the inverse proportionality equation y=k/x
so first set it up as 15(people) = k(constant) / 4(half the items means half the time)
solve for k and you get k = 60
now just put in the new number of people and you have 6 = 60/x(the new hours)
solve for x and x = 10, the number of hours it takes 6 people to make half the items</p>