The perimeter is 24 inches. a poster is 3 times as long as it is wide. what are the length and width of the poster.

In this question the main information's are already provided. Based on them the answer to the question can be easily deduced.
Let us assume the width of the poster = x
Then the length of the rectangle = 3x
The perimeter of the rectangle = 24 inches
we already know
Perimeter = 2 ( Length + width)
24 = 2( 3x + x)
24 = 2 * 4x
24 = 8x
x = 24/8
  = 3
So the width of the rectangle is 3 inches
Length of the rectangle = 3x
                                     = 3 * 3
                                     = 9
So the length of the rectangle is 9 inches.

Maybe the answer is 20mm. all i really did was just multiply, which is obvious.