SOLUTION: A rectangular box has dimensions 4ft by 5ft by 5ft. Increasing each dimension of the box by the same amount yields a new box with volume seven times the old. Find how much each dim

Question 274582: A rectangular box has dimensions 4ft by 5ft by 5ft. Increasing each dimension of the box by the same amount yields a new box with volume seven times the old. Find how much each dimension of the orginal box was increased to create the new box. Round your answer to two decimal places.
This is where I started:
0= x^3+20x^2 ---->this part isn't finished, I don't understand how to get it...please help.

Thank You!
Answer by Alan3354(69443) (Show Source): You can put this solution on YOUR website!
A rectangular box has dimensions 4ft by 5ft by 5ft. Increasing each dimension of the box by the same amount yields a new box with volume seven times the old. Find how much each dimension of the orginal box was increased to create the new box. Round your answer to two decimal places.
This is where I started:
0= x^3+20x^2 ---->this part isn't finished, I don't understand how to get it...please help.
----------------------
The volume is 4*5*5 = 100 cubic feet.
The increased volume is (4+x)*(5+x)*(5+x) = 700
x^3 + 14x^2 + 65x + 100 = 700
x^3 + 14x^2 + 65x - 600 = 0
x = 4.225 (by graphing)
--> 4.23 to 2 decimals

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