Question

A rectangle's width is twice as long as its length. If the perimeter is 36 in., what is the length?

Answer 1

Okay here: 

Let L be the length and W the width of the rectangle. 

1. w=2•L
2. 2L+2W=36

Substitute eq. 1 into eq. 2, 

2L+2•(2•L)=36

2L+4L=36

6L=36

L=9

Not done yet, then from eq. 1, 
 
W=2•L
W=2•9
W=18 <------- Your answer. :)