Find the Four IntegersDate: 06/10/2003 at 11:51:33 From: Rabbit Subject: A mathcounts problem. Four distinct integers a, b, c, and d, have the property that when added in pairs the sums 16,19,20,21,22, and 25 are obtained. What are the four integers? Is there any way to do this problem without having to guess and check? Is there a shortcut? Date: 06/10/2003 at 13:51:15 From: Doctor Peterson Subject: Re: A mathcounts problem. Hi, Rabbit. Here is a similar, but considerably more complicated, problem: Weighing Bales of Hay http://mathforum.org/library/drmath/view/56767.html We can use similar techniques in your problem. In most such problems there will be some guessing and checking, but it is orderly and not wasteful. First, we know that the two smallest numbers must add up to 16, and the two largest to 25. We can make an addition table, assuming that a<b<c<d: a b c d --- --- --- --- a| - 16 ? ? | b| - ? ? | c| - 25 | d| - Think about the order of the sums. Clearly a+b < a+c and b+d < c+d; so we can indicate relationships this way: a b c d --- --- --- --- a| - 16 < ? < ? | < < b| - ? < ? | < c| - 25 | d| - We don't know how b+c and a+d are related; but we know enough to fill in a+c and b+d: a b c d --- --- --- --- a| - 16 < 19< ? | < < b| - ? < 22 | < c| - 25 | d| - This tells us that c = b+3. There are two ways to fill in the last two spaces; this tells us that b-a = 1 or 2, and d-c = 1 or 2: 1 3 2 2 3 1 a b c d or a b c d --- --- --- --- --- --- --- --- a| - 16 < 19< 21 a| - 16 < 19< 20 | < < | < < b| - 20< 22 b| - 21< 22 | < | < c| - 25 c| - 25 | | d| - d| - But since a+b is even, their difference can't be odd; do you see why? Knowing that a+b=16 and b-a=2, we can easily finish the problem. One thing that might help is to "play" with my ideas before trying to work through the logic. That is something I often do when I approach a new problem. In this case, that means picking some numbers and seeing what sums you get, in order to get a feel for how the sums are related to the numbers you start with. The problem involves working backward with that idea, so it's a good idea to start by seeing how it works forward. Let's take a set of four numbers at random; say, 6, 8, 11, and 14. We can look at all their sums by making an addition table like this: + 6 8 11 14 --- --- --- --- 6| 12 14 17 20 | 8| 14 16 19 22 | 11| 17 19 22 25 | 14| 20 22 25 28 One thing to notice is that the table is symmetrical; if you draw a diagonal from the top left to the bottom right, the numbers "reflect" in that "mirror," so that the 14 is opposite another 14, and so on. That's because 6+8 = 8+6; we say that addition is commutative. So in order to see all possible sums of pairs, we can ignore half of the table, which is a duplicate of the other half: 6 8 11 14 --- --- --- --- 6| 12 14 17 20 | 8| 16 19 22 | 11| 22 25 | 14| 28 Notice also that the sums on the diagonal are a number plus itself, and we are not allowing that in the problem; so we can remove those: 6 8 11 14 --- --- --- --- 6| - 14 17 20 | 8| - 19 22 | 11| - 25 | 14| - Now you can notice some things about the rows. Look at the first row: the numbers are in order, because you are adding bigger and bigger numbers to 6. The same is true in every row, and in every column. And it will always be true. That's why in my answer I put "less than" signs into the table, like this: 6 8 11 14 --- --- --- --- 6| - 14 <17 <20 | < < 8| - 19 <22 | < 11| - 25 | 14| - This means that 14 < 17 < 20 19 < 22 17 < 19 20 < 22 < 25 When I worked backward, I knew that these relationships had to be true, without having filled in the numbers! And that was the key to my solution. I hope this fills in some background so you can better understand my thinking. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/ Date: 06/12/2003 at 11:48:50 From: Rabbit Subject: A mathcounts problem. What I don't understand is how you know that b-a = a or 2, and d-c = 1 or 2 (it was in the first part of the discussion). Thanks! Date: 06/12/2003 at 12:03:39 From: Doctor Peterson Subject: Re: A mathcounts problem. Hi, Rabbit. Here is the table I came up with: a b c d --- --- --- --- a| - 16 < 19< ? | < < b| - ? < 22 | < c| - 25 | d| - I said that the difference from 16 to 19 tells us that c = b+3. Then there are two numbers left to fill in the two missing spaces, which can be done in two ways: a b c d or a b c d --- --- --- --- --- --- --- --- a| - 16 < 19< 21 a| - 16 < 19< 20 | < < | < < b| - 20< 22 b| - 21< 22 | < | < c| - 25 c| - 25 | | d| - d| - The first of these shows that changing from a to b (going from the first to the second row) adds 1 (19 to 20, 21 to 22). So b must be 1 more than a. Similarly, the second goes from 19 to 21 and 20 to 22, so b must be 2 more than a. You can do the same with the last two columns to show that d is 1 or 2 more than c. Does that help? - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/ Date: 06/12/2003 at 12:09:38 From: Rabbit Subject: Thank you (A mathcounts problem.) Now I understand. Thank you so much for taking the time to help me, Doctor Peterson! |
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