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Finding missing side length when given perimeter

CCSS.Math:

Video transcript

the perimeter of the figure is 24 centimeters what is the length of the missing side so we're told this figure down here has a perimeter of 24 centimeters the perimeter is 24 centimeters so what that tells us is that the distance around the entire outside is 24 centimeters but what we want to know is what is the length of the missing side well the missing side is going to be the side we're not told so it will be this one here because we have this question mark it's missing it is not labeled so we've got to figure out what number should go on this missing side so that the entire length is 24 centimeters so let's start with what we do know we know that this top length this top side has a length of 6 centimeters plus going down the left has a length of 5 more centimeters plus across the bottom is 4 more centimeters and the last known side is another 4 centimeters so when we add all those Plus this mystery side we should get a perimeter of 24 the entire distance all of this the 6 plus the 5 plus the 4 plus the 4 Plus this mystery number should get us to a perimeter of 24 so let's add over here and try to make it a little simpler to find our missing side I can add any order and I love making tents so I'm going to say 6 + 4 is 10 10 plus 5 is 15 and 15 plus 4 more is 19 so we know we're 19 centimeters come from on this shape here's the 19 centimeters are here outlined in blue so how many more centimeters what length must this missing side be to get us a total distance around the outside of 24 centimeters we can do that math 19 plus 1 more is 20 and then plus four more to twenty four so one more to twenty and four more that's a total of five more so the missing side length is five centimeters this side right here must be five centimeters in order for the total perimeter to be twenty-four okay let's look at one more this time it says that the perimeter of a rectangle is 32 meters and one side is 11 meters what is the length of the missing side so again we have a missing side so let's visualize this let's draw ourselves a rectangle here this is what a rectangle looks like and we know that one side is 11 centimeters so one side is 11 or meter excuse me and because we know that this side is 11 meters we also know that this side is 11 meters on a rectangle opposite sides are equal so those two we know but what we don't know are this top and the bottom those are the missing sides so what are those side links well again we know we have a perimeter of 32 meters we're told right here perimeter is 32 meters so the question is 11 plus 11 for these for these two sides 11 and 11 plus some mystery number and then Plus that same mystery number because these two sides are equal and that has to give us a total a total of equals 32 because 32 is the total distance around the outside so let's add the Elevens 11 plus 11 is 22 so 22 Plus how many more gets us to 32 well 22 Plus 10 more is 32 but if we label one of these sides is 10 then the other one would have to be zero so they have to be equal so how can we equally split those ten meters between the two sides half of 10 is 5 so these sides are each 5 meters 20 to 5 plus 5 is 10 22 plus 10 is 32 so the missing side length is 5 meters