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Current time:0:00Total duration:3:14

CCSS.Math:

mic built a rectangular dog pen that is 21 feet long and has a perimeter of 78 feet what is the width of Mike's dog pen so let's draw what it might look like so it might look something like this do my best to draw it a rectangular potential dog pen so let's say that this is the dog pen right over here and they give us its length its length is 21 feet its length is 21 feet so this distance right over here is 21 feet and of course that distance is 21 feet this distance is also this distance is also going to be 21 feet now they also tell us that the perimeter is 78 feet so the perimeter is 78 feet 78 feet so we need to figure out the width and the width would be the width would be this length or hide this width I should say this width which is going to be the same thing as that width right over here so let's think about and we know how we figure out the perimeter if we added this distance 21 feet to this distance the width plus 21 feet to the width again we're going to get 278 feet so one way to back into what the width is is to say well look if we sum up all of these lengths we're gonna get 28 feet so let's at least subtract these two orange lengths the two 21 feet from the 78 feet and then we figure out how much length essentially these two widths have to make up so let's subtract let's do 78 minus 21 minus 21 and what's that going to be equal to so that's 78 minus 42 and 78 minus 42 I want to make a careless mistake 78 minus 42 is going to get me to 36 so this is equal to 36 so let's remind ourselves what 36 is what 36 feet is that's the total length or I should say the total width if we were to take the width down here and add it to the width up here now we know something else these are the exact same width that this distance is the same as the distance and if we add them together we get to 36 so what are two distances or what are two numbers that I can add together to get to 36 well we know that 18 plus 18 we know that 18 plus 18 is equal to 36 so this one down here must be 18 and this one up here must also be 18 and we can verify that try adding all of these up together you add the two widths you get 18 plus 18 and you add the two lengths plus 21 plus 21 and you are left with 36 which is exactly what we got here this is what we needed the two widths to add up to plus 42 plus 42 which is of course equal to 36 plus 42 is equal to 78 which is the perimeter