SOLUTION: i don't understand how to do this? can you please help? here is the problem: a poster is 20 cm longer than it is wide. it is mounted on a piece of cardboard so that there is a 10

Question 179491: i don't understand how to do this? can you please help? here is the problem:
a poster is 20 cm longer than it is wide. it is mounted on a piece of cardboard so that there is a 10 cm border on all sides. if the area of the border is 2400 cm squared what is the area of the poster?
Answer by nerdybill(7384) (Show Source): You can put this solution on YOUR website!
a poster is 20 cm longer than it is wide. it is mounted on a piece of cardboard so that there is a 10 cm border on all sides. if the area of the border is 2400 cm squared what is the area of the poster?
.
Let x = width of poster
then
x+20 = length of poster
.
Cardboard backing is:
x+10 = width
x+20 + 10 = length
.
"border area" = "cardboard area" - "poster area"
2400 = (x+10)(x+30) - x(x+20)
2400 = x^2+30x+10x+300 - x^2-20x
2400 = x^2+40x+300 - x^2-20x
2400 = 40x+300 -20x
2400 = 20x+300
2100 = 20x
105 cm = x (width)
.
length:
x+20 = 105+20 = 125 cm
.
area = 105*125 = 13125 square cm

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