Calculate Rectangle Perimeter: Finding the Border Length of EFCD (3×5)

Look at the following rectangle:

What is the perimeter of the rectangle EFCD?

Since every pair of opposite sides are equal in a rectangle, we know that:

$BC=AD=5$

Now we can calculate side ED:

$AD-AE=ED$

Let's substitute in the known data:

$5-3=ED$

$ED=2$

Side ED is equal to side FC and therefore also equal to 2.

We also know that:

$AB=CD=EF=7$

Now we can calculate the perimeter of rectangle EFCD:

$2+7+2+7=4+14=18$