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

Algebra ->  Polynomials-and-rational-expressions -> 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      Log On


   

Question 274690: 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 JBarnum(2146) About Me  (Show Source):
You can put this solution on YOUR website!
4%2A5%2A5=100
%284%2Bx%29%285%2Bx%29%285%2Bx%29=7%28100%29
%284%2Bx%29%2825%2B5x%2B5x%2Bx%5E2%29=700
%284%2Bx%29%2825%2B10x%2Bx%5E2%29=700
%28100%2B40x%2B4x%5E2%29%2B%2825x%2B10x%5E2%2Bx%5E3%29=700
100%2B65x%2B14x%5E2%2Bx%5E3=700
-600%2B65x%2B14x%5E2%2Bx%5E3=0
x%5E3%2B14x%5E2%2B65x-600=0
x%5E2%28x%2B14%29%2B5%2813x-120%29=0
x=-14,x=120/13 since x cant be a negative number, then x = 9.230769repeating
5%2Bx=9.23=4.23
each was increased by highlight%284.23%29%29
check
8.23%2A9.23%2A9.23=701 if it was increased by 4.22 it would=698
hmmm for some reason i failed to calculate things correctly but by doing guess and check i came to the conclusion of 4.23