To find the perimeter of a rectangle, the formula is 2*length+2*width=perimeter.
So, say that the variable x is the length. Because the width is 3 times the length x, the width is equal to 3x.
I can plug what we know into the formula.
(2*x) + (2*3x) = 40
2x + 6x = 40
8x = 40
Divide both sides by 8 to get x = 5.
So, your length is 5 inches!
(To double check, you can just plug it into the problem. (2*5) + (2*15) = 10 + 30 = 40.)
