# Length, Width, and Perimeter | 5th GRADE Math Review

Time for another happy studying session with Mometrix Academy. Hi, and welcome to this video lesson on width, length, and perimeter of parallelograms.

Sometimes in math we’re asked to find the width, length, or perimeter of a parallelogram, which is kind of like a rectangle, or kind of like this. Yeah, that wasn’t very good. When we’re trying to find unknown widths or lengths of parallelograms, it helps us to be able to use equations.

This isn’t as hard as it sounds, let’s take a look at an example. Say we have a rectangle with length, a, and width, b. Describe the perimeter and write an equation to represent the perimeter of the rectangle. The perimeter of a rectangle is the sum of the length of the sides of the rectangle. The perimeter of the rectangle can be shown as a plus a, plus b, plus b, or 2a plus 2b.

Now that we know the equation that will be the key to solving any perimeter problem we have, let’s start adding known numbers to this equation. First, say we are given this picture. In this picture we are given two things, the perimeter and a side, so let’s set up our equation. Perimeter is equal to 2A + 2B, right?

Let’s put in our known quantities. 34 is our perimeter, and that is equal to 2 times A, or in this case, 7, plus 2B. We can simplify this a little bit more. 34 is equal to 14 plus 2B. Now, let’s go ahead and get the unknown B by itself, by subtracting 14 from both sides of this equation. 14 from 34 is equal to 20, and 14 away from 14 plus 2B, is equal to just 2B.

Now we have 20 is equal to 2B. Let’s go ahead and solve the rest of this equation, dividing by two on each side. 20 divided by two is equal to 10, 2B divided by 2 is equal to B. Now we have B alone, and we solve for B. Now we know the lengths of all the sides on this rectangle. Let’s try one more, this time by yourself.

The perimeter is 26 and we have sides x and 3. Remember, set up your equation, then attempt to get the unknown alone by doing the same thing to both sides. Remember the width on this rectangle is 3 on each side, and the length is x on each side, which means we have 2(3) plus 2x is equal to the perimeter of 26. Let’s go ahead and simplify this equation.

Now we have to get x alone, that requires some division, so let’s go ahead and do that. Finally, we see that x is equal to 10 once more. I hope that helps. Thanks for watching this video lesson, and until next time, happy studying. Yeah, really gonna get rid of that.