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Analyzing structure word problem: pet store (1 of 2)

Sal solves a word problem about the unknown number of bears, cats, and dogs in a pet store. This is part 1 where Sal uses a visual reasoning. Created by Sal Khan.

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  • leafers ultimate style avatar for user Ghost
    Could you not also input the lowest possible numbers like 1 bear over 3 cats 2 dogs and 1 bear ( 1b / 3c+2d+1b) to figure out an answer depending on what yo wanted to find. so 1 bear would be 1 over 6 or would the 1 bear on the numerator cancel out the bear on the denominator?
    (25 votes)
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  • piceratops ultimate style avatar for user Bilal Salam
    I don't think that it makes any logical sense. Think of it as a common sense problem. Can someone explain?
    (9 votes)
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    • mr pants teal style avatar for user IanS
      Here is how I thought about it. 1/3 is the same as 1/1+1+1 and we can replace 1 with a variable that represents an arbitrary number of units. In this case if we went with b like the problem in the video, we would get a ratio of b/b+b+b in order to be equivalent to 1b/3b, or 1/3 when simplified.

      Though since d>b and c>d, we know that the denominator, c+d+b, has to be larger than 3b and when you increase the denominator while keeping the numerator the same you end up with a smaller fraction - for example 1/3 compared to 1/6, which is the largest possible ratio that b/c+d+b could be if we went with the least possible number of animals (1b, 2d, 3c).
      (17 votes)
  • female robot grace style avatar for user Bella
    that's only true if a, b, and d were all consecutive integers (odd or even).
    because a>d>b could be 5>2>1 which is 2/ 8 which is 1/4 not 1/3
    ?
    (5 votes)
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    • piceratops ultimate style avatar for user sahsan
      Well, you have answered your question yourself here, the question is not an equality statement but rather a question of what is bigger or smaller to another thing. In this case, c>d>b, you suggest the combination 5>2>1 the equation is b/c+d+b which would be 1/1+2+5 = 1/8, and 1/8 is definitely smaller than 1/3. Hope that helps :)
      (17 votes)
  • female robot ada style avatar for user Lyndsey De Armas
    Where can I find a video that shows how to plot linear inequalities for coordinates?? Thanks
    (5 votes)
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  • duskpin ultimate style avatar for user Rabi
    Can I say that,

    since c > d > b, then c+d >> b, (">> = much greater")

    and since we have c+d+b in the denominator this will be

    c+d+b >> c+d , and therefore b/c+d+b < 1b/3b .

    Since the c+d+b in the denominator, there for the value will be smaller .

    Thank you
    (3 votes)
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  • piceratops ultimate style avatar for user AKSHAI
    How can we tell how much greater the #dogs is to the # bears?
    (2 votes)
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  • duskpin ultimate style avatar for user Taly
    We have dogs(D) and cats(C). There are more dogs than cats and there are at least one cat.

    Compare the expressions C/C+D and 0.4. Which statement is correct? =, > or <

    The correct answer say there are not enough informations.

    But i want to know how to express the thought that its not C/C+D = 2/5? are they the same ratio?

    My thought was that the two expressions convey the same meaning and i realize that the quantity of the variables might be different.
    (2 votes)
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    • stelly blue style avatar for user Kim Seidel
      I usually assess these types of problems by trying different values for the variables.
      Your problem tells you there is at least 1 cat. So, C = 1, 2, 3, 4, etc.
      And, there are more dogs than cats.
      The only way to get c/(c+d) = 2/5 would be for c=2 and d=3 (or multiples of these)
      But, you could do c=1; d=2. In this case: c/(c+d) = 1/3 (smaller than 2/5)
      And, we could also do c=4; d=5. In this case: c/(c+d) = 4/9 = 0.444... with 4 repeating. So, it is now larger than 2/5.
      Hopefully you can see from these examples that depending on the values of C and D, all the possibilities are correct. This is why there is not enough info.

      Hope this helps.
      (3 votes)
  • male robot donald style avatar for user AidianConnelly
    Who else noticed the name of the title? Dogs cats, and bears? I think Sal meant Dogs, cats, and bears.
    (3 votes)
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  • duskpin ultimate style avatar for user Goat
    If you just wanted to simplify the fraction of animals that are bears, and you cancelled out the b in the top and bottom, would the numerator become a 1, or would it be a 0, and cease to be a fraction?
    (1 vote)
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  • male robot hal style avatar for user richmasterninja
    Suppose i substituted values 1, 2, and 3 respectively for b,d and c. You get the minimum as 1/6, as the values must be integers. Would it be correct?
    (2 votes)
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Video transcript

Let's say that someone's running a fairly unusual pet store that only has three types of animals in it. It has cats, and we represent the number of cats with the letter c. It has dogs, and we represent the number of dogs by the letter d. And it has bears. That's what makes it unusual. And we're going to represent the number of bears with the letter b. Now, this person also tells us that at this unusual pet store, the number of cats is greater than the number of dogs, which is greater than the number of bears. Now based just on this information, they then ask us which expression is greater. b divided by c plus d plus b or 1/3. And I encourage you to pause the video now and come up with your own rationale behind which of these is greater. Or maybe neither is greater. Maybe they're equal. Or maybe you just don't have enough information from what this pet store owner told us to actually figure out. So pause the video now. So let's reason through what this expression is actually representing. This is b, the number of bears, over the number of cats, plus the number of dogs, plus the number of bears. So there's the number of bears over the sum of all the animals. So this really is the fraction that are bears. Now so this question really reduces to is the fraction that are bears greater than a third, less than a third, equal to a third, or can we not tell? And there's a bunch of different ways to do this problem. And I'll try to expose you to many ways in this video and the next. So let's do a visual one doing different cases. Let's do the scenario where the fraction that are bears are greater than one third. So let's visualize this. So this little diagram right over here. Let's say this is all of the pets in the pet store. And I've divided it into thirds right over here. If the fraction that are bears is greater than one third, then it might look something like this. So this is a third under the dotted lines. If I want to be greater than 1/3, I'll go a little bit more than greater than 1/3. So we go greater than 1/3 right over there. But if the fraction of bears is greater than 1/3, well the fraction of dogs has to be even greater than that. So it has to be at least as much as this blue area. So it's going to be even greater than that. And the fraction that are cats has to be even greater than that. And I didn't even make this one that great. It has to be more than this blue area. And the fraction that are cats has to be even greater than that. And you see, you can't have three things that are all greater than 1/3 adding up to a whole. You can't have all three parts of it being greater than 1/3. So this scenario breaks down. Another way you could have thought about it is if this expression right over here is greater than a third, then c plus d over this has to be less than 2/3. But that means one of them, c or d, would have to be definitely less than a third in order for that to work out. So at least thinking about this way, you know that this thing cannot be greater than a third. Let's think about whether it can be equal a third. Let's imagine if this expression, the fraction that are bears, were equal to a third. So let's make our diagram again. So in this situation, the bears are exactly 1/3. So I'll just color in this third right over here. So exactly 1/3 are bears. But we know that the number of dogs is greater than the number of bears. So the fraction that are dogs has to be greater than the fraction that are bears. So the fraction that are dogs would have to be greater than 1/3. But if the fraction that are dogs are greater than 1/3, then all we have left is something less than 1/3 for the cats. That's actually the largest fraction of the animals are cats. So we know that this also is not a possibility. So what we're really left with is that this has to be less than 1/3. And let's make sure that's reasonable. So, let's paste another example right over here. So if the bears are less than 1/3-- let me make it a little bit more dramatic, so that you can make it clear. So this is less than a third. let's say that the purple, the dogs, let's say they're roughly exactly 1/3. Then you could make a scenario where you have the largest proportion of cats. So this would be the cats right over here. That's not the same color as the cats. Let me do the same color as that pink color. So this right over here is our cats. So this is complete believable. c over c plus d plus b does indeed look bigger than d over c plus d plus b, which does indeed look bigger than b over c plus d plus b. This is completely believable to be less than 1/3. So based on just this visual argument that we just made, you could say hey, look. This right over here is the larger quantity. In the next video, I'll give what I would call more of an analytical argument, where I won't draw diagrams to show that this has to be less than 1/3.