Analyzing structure word problem: pet store (2 ofÂ 2)

Common Core Math:

Video transcript

In the last video, we made a visual argument as to why this expression has to be less than 1/3, and this expression we already figured out is the fraction that are bears. Now we will make an algebraic argument, or I could call it an analytic argument. And to make this argument, I'm going to leave this expression-- we know this is the fraction that are bears-- and I'm going to write this 1/3 in a form that looks a lot like this, and then based on the information we have, we can directly compare them. So how can I write 1/3? Maybe with the b as a numerator. Well, 1/3 is the same thing as b over 3b, which is the exact same thing as b over b plus b plus b. So now, this is looking pretty similar. The only difference between this expression right over here, b over c plus d plus b and b over b plus b plus b is that our denominators are different. And the only difference in our denominators, this denominator has a c plus d here, while this has a b plus b over here. Now, we have to ask ourselves a question. What is larger? Is c plus d larger than b plus b? And I encourage you to pause that and think about that for a second. Well, yes. We already see right over here. It was given to us that c is greater than d that is greater than b, so both c and d are greater than b. So c plus d is definitely going to be greater than b plus b. So this denominator right over here is greater, so this has a larger denominator. This right over here has a smaller denominator. And since we know this has a larger denominator, this has a smaller denominator, they have the exact same numerator-- they both have b as a numerator-- we know that this whole thing must be a smaller quantity. If you have the same numerator but one expression has a larger denominator, it must be smaller. Wait, so how does that work? Well, just remember. I mean, just imagine. You have the same numerator, what's going to be bigger, a over 7 or a over 5? Well here, you're dividing a by 7. You're dividing into many more chunks than over here, so this right over here is smaller. This right over here is larger. So this is the larger. This right over here is smaller. So the same numerator, the larger the denominator, the smaller the quantity is going to be. So going back to the original question, this is the smaller quantity, and this right over here, 1/3, is the larger quantity.