# Department of Science & Math

E = mc²

Science journals of interest: Scientific American - Nature - New Scientist - Science AAAS - Science Daily

LAST EDITED: March 17, 2015

Here are some interesting links I had stickied:
Yeano's thread: Proving Something is Independent of our Axioms.
Yeano's thread: Algebraic Topology and Model Theory

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I need help smart people!
Posted: Posted August 10th by Brandy
 The variables x and y are directly proportional, and y = 2 when x = 3. What is the value of y when x = 9 ? Please help!
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There are 16 Replies

This is what I would do...

3y = 6
2x = 6

Therefore:
3y = 2x

Plug in 9 for x:
3y = 2(9)
3y = 18
y = 6

I think that works? I have a learning disability that makes me real bad at math so you might wanna get a second opinion.

Posted August 10th by Cetasaurus
Cetasaurus
formerly KM8

Thank you! Anyone can double check that?

Posted August 10th by Brandy
Brandy

Choose the best answer from the four choices given. You may use scratch paper.

x-1
If ———— = 20, then x =
x

Posted August 10th by Brandy
Brandy

A group of 18 people ordered soup and sandwiches for lunch. Each person in the group had either one soup or one sandwich. The sandwiches cost $7.75 each and the soups cost$4.50 each. If the total cost of all 18 lunches was $113.50, how many sandwiches were ordered? Posted August 10th by Brandy Brandy s = # of sandwiches p = # of soups s(7.75) + p(4.50) = 113.50 s + p = 18 p = 18 - s s(7.75) + (18 - s)(4.50) = 113.50 s(7.75) + [(18)(4.50) - (s)(4.50)] = 113.50 s(7.75) + 81 - (s)(4.50) = 113.50 s(7.75) - (s)(4.50) = 32.5 s(3.25) = 32.5 s = 10 Posted August 10th by Cetasaurus Cetasaurus formerly KM8 A group of 18 people ordered soup and sandwiches for lunch. Each person in the group had either one soup or one sandwich. The sandwiches cost$7.75 each and the soups cost $4.50 each. If the total cost of all 18 lunches was$113.50, how many sandwiches were ordered?

Ooh that's a tricky one. You can at least get a range for it...

• You know that you have 18 people
• You know that they each had one item, a soup or sandwich
• You know the cost of sandwiches and soups
• You know the total spent

So if everyone ordered sandwiches that would be (18 people)*($7.75), which is$139.50.

If everyone ordered soups, that would be (18 people)*($4.50), which is$81.00

You know it's somewhere in between there. The "Everyone had a sandwich" is closer to your goal, so you can start with that.

Then you look at the difference between the soup price and the sandwich price ($7.75-$4.50) = $3.25. So you should be able to start with a sandwich for everyone =$139.50 and then subtract $3.25 a bunch of times until you get to your$113.50 answer. Each time you subtract $3.25 you're trading a sandwich for a soup because of the price difference between them. If you want to do this with math, you could look at the difference between them...$139.50 - $113.50 =$26.00

then divide that by $3.25 to get the number of soups... Which I got a decimal number so I obviously messed up somewhere, but at least now you have a way to solve the problem :) • Edited August 10th by Xhin Xhin Also yeah you should probably use algebra like ceta said. I've been a programmer for so long I just reinvent math when I need to solve a problem. Posted August 10th by Xhin Xhin Yall are awesome, thank you. Posted August 10th by Brandy Brandy Anytime. :) Are you studying for some kind of standardized test? Edited August 10th by Cetasaurus Cetasaurus formerly KM8 Yes it's called a TSI test Posted August 10th by Brandy Brandy Good luck. :) I've been studying for the GRE myself. Let me know if I can be of any more help (in any of the subjects covered). Posted August 10th by Cetasaurus Cetasaurus formerly KM8 If X and Y are proportional, then both are the same at 0. Find out the relationship between the numbers. 2 is 2/3 of 3. So: y = x*2/3, or x*.6666 And if X = 9: Y = 9*(2/3) Y = 6 Posted August 12th by mariomguy mariomguy the answer y = mxb4 Posted August 12th by poptart! poptart! I hereby confirm the answer given by Cetasaurus: 10 sandwiches and 8 soups. Algebra is a savior! Love you all for keeping this forum alive. Posted August 14th by The Fly The Fly Hey you guys and gals, did you know that you can actually get the same answer to the sandwich-soup problem even without knowing that there were 18 people!!? (The total cost$113.5 puts a limit on how many people there were.) Can you solve it without the 18, and instead in fact derive it? So: how many people were they in total and how many sandwiches did they order?

Edited August 14th by The Fly
The Fly

Xhin
You are right with your solution: $26 /$3.25 = 8, which is the number of soups
You just messed up your last step, everything else was completely correct.

Posted September 27th by Jo
Jo

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