<p>I am having trouble with combinations and permutations. I get what they mean. Comb are calculating at any order and perm is in order. But what troubles me are the questions. I cant figure out if its a combination or permutation they want me to calculate. Is there any keywords in the Level 2 questions?</p>
<p>I'm also confused with finding avg speeds. PR taught me the formula is
avg speed = total distance/ total time
to find time its T=distance / rate</p>
<p>Here are some exercise they showed. I would like to see the algebraic work before showing me some shortcuts(if apply)</p>
<p>A truck travels 50 miles from Town S to Town T in 50 min, and then immediatlely drives 40 miles from Town T to Town U in 40 min. What is the truck's avg spped in miles per hour from Town S to Town U?
Answer: 60</p>
<p>Ben travels a certain distance at 25 miles per hour and returns across the same distance at 50 miles per hour. What is his average speed in miles per hour for the round trip?
answer: 33.3</p>
<p>Well you could use common sense for the 1st one. 50miles/50min = 60mph and 40miles/40min is also 60mph...average the two, it's 60mph. and for 2, just do (2<em>25</em>50)/(25+50)</p>
<p>permutations means order matters, ex. locker combinations.</p>
<p>combinations- they don't matter, for example, getting 5 cards in a hand.</p>
<p>For the r= d/t, do the chart. It's too long to explain here, but google how to solve rate time distance problems and something will come up and explain it.</p>
<p>Given your formula avg speed = total distance/total time, for the first one you would add up the miles to get the total distance (50 + 40 = 90 miles) and divide that by the time in hours 50 mins + 40 mins = 90 minutes/60 minutes in an hour = 1.5 hours. 90 miles/1.5 hours = 60 mph</p>
<p>On the second one you can shortcut it by taking the harmonic mean of the two numbers. For two numbers 'a' and 'b' the harmonic mean is (2<em>a</em>b)/(a+b). Therefore in this problem (2<em>25</em>50)/(25+50) = 2500/75 = 33.33</p>
<p>Don't ask me why the harmonic mean thing works, but it has served me well in mathletics for a few years now.</p>
<p>I wish I could help with combinations/permutations but that's something I need to review before saturday too :(</p>
<p>I still dont get about the com and perm. Maybe ill reword my question "how can you tell if the question is asking for perm or com to solve it"</p>
<p>I have this question in Meylani</p>
<p><---A-B-C-D-E-F-G--->m
Two of the seven points on the line m above are to be selected and connected to amke a line segment. How many unie line segments can be constructed?
I first assume its permutation because it did say unique as in it has to be ordered, i think.
so in my calc 7 P 2 = 42 but the answer is 21 from 7 C 2.</p>
<p>It is combination because line segment AB=BA. They are the same segment.</p>
<p>I think of combinations as basketball teams. Any five people can go on the court at once but it doesn't matter what order they go on the court. If I have Players A, B, C, D, and E on the court, it is the same as having E, D, C, B, and A.</p>
<p>Permutations are more like a batting order. Player x batting first and player y batting second is different the having player x batting second and player y batting third.</p>
<p>on that one i would just do 6+5+4+3+2+1, starting at A and counting how many letters can be the endpoint. 6 between A and b,c,d,e,f,g. 5 between B and c,d,e,f,g, etc. :s</p>
<p>If it asks for a group of people or an outfit, a pizza w/ toppings etc (anything with a group of things together)-- Combination because it doesnt matter the order in which things are picked to be in a group</p>
<p>If something is a line, a word, etc (aka order matters)--Permutation</p>
<p>Basically you have to think whether or not AB is the same as BA in terms of the question.</p>