Who Picked the Most?
Date: 09/12/98 at 00:27:23 From: Will Subject: Summer Vacation During summer vacation, Anita, Brent, Carlo, and Donna worked together picking peaches. At the end of the season, they compared the number of peaches that they each had picked. They made note of the following facts: 1. Brent picked more than Anita and Carlos picked together. 2. The total of what Anita and Brent picked just equaled the number Carlos and Donna picked together. 3. The total picked by Donna and Carlos was more than the total picked by Brent and Carlos. Arrange the names of the people in the order of the number of peaches that each picked, starting with the person who picked the most. The answer I have is that Donna picked the most because of fact number 3. Brent came in second because of fact number 1. But the part that I have trouble with is finding who came in 3rd and who picked the fewest peaches, Anita or Carlos.
Date: 09/12/98 at 10:34:04 From: Doctor Kate Subject: Re: Summer Vacation Will: I would find this problem very difficult to solve in my head. Instead, I'm going to try a very useful translation technique. Let's call the amounts Anita, Brent, Carlos and Donna picked "A", "B", "C", and "D" for short. Now let's translate the clues. 1. Brent picked more than Anita and Carlos picked together. That is, the amount Brent picked, B, is more than what Anita and Carlos picked together, A + C. (Do you see why I've added A and C? If A is what Anita picked, and C is what Carlos picked, A + C is what they've picked together.) So this clue really means: B > A + C Now that you see what I'm doing, let's translate the others. You try it first, and then see if you get the same thing I get. 2. The total of what Anita and Brent picked just equaled the amount Carlos and Donna picked together: A + B = C + D 3. The total picked by Donna and Carlos was more than the total picked by Brent and Carlos: D + C > B + C Okay, so what does that do for us? Well, it gives us three pieces of information in the form of equations and inequalities. We can combine these bits of information mathematically. Here's where all the algebra comes in. First, notice that if D + C > B + C , we can remove C from both sides and find that D > B. In plain English, if Donna and Carlos picked more together than Brent and Carlos did, Donna must have picked more than Brent. So Donna's looking good as a winner, but we don't know that quite yet. We don't know anything about Anita and we aren't sure where Carlos is. Let's do some more algebra. We also know B > A + C. This means that B > A and B > C because we know that A and C are both zero or positive numbers. This one makes most sense in english. If Brent picked more than Anita and Carlos combined, surely he picked more than Anita and more than Carlos separately. So we have D > B, B > A and B > C. So the only thing left to know is whether A > C, A = C or A < C. That's where you got stuck. This part is a little more tricky, and we have to rely more on the algebra instead of the mathematics. A good idea to find out where to start is to ask "What information have I not used?" and the answer is that we haven't used the fact that A + B = C + D. So let's try to use this information. Well, looking at it by itself, it doesn't tell us whether A or C is bigger, but we are forgetting what we already know. That is, that D > B. Now, in algebra, we say that: A + B = C + D and C + D > C + B So we know A + B > C + B. We can use the same trick as before and get rid of the B, finding that A > C. And we're done. What does it mean? Well, this is a little trickier in English, but I could say it like this: We know that Anita and Brent picked the same as Carlos and Donna, but Donna picked more than Brent. So Anita must have picked more than Carlos in order to make the pairs equal. It's hard to imagine in English, and it's worth the effort of trying, but later in mathematics you'll encounter all sorts of problems that no one can do in English, but the Mathematical methods will be able to solve. So being able to do this problem both ways is important. So please let me know if you have any questions. - Doctor Kate, The Math Forum http://mathforum.org/dr.math/
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994- The Math Forum at NCTM. All rights reserved.