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 March 21st, 2010, 07:44 AM #1 Newbie   Joined: Mar 2010 Posts: 10 Thanks: 0 math question (cube, cube volume, percent) I can't solve this. I need help. The question reads this: On how much percent would the volume of a cube increase if the length of each of its 12 sides increased by %20? Any thoughts would be highly appreciated.
 March 21st, 2010, 11:08 AM #2 Senior Member   Joined: May 2008 From: York, UK Posts: 1,300 Thanks: 0 Re: math question (cube, cube volume, percent) The volume of a cube is equal to the product of the lengths of any 3 mutually perpendicular sides of the cube. Does this help?
 March 21st, 2010, 07:53 PM #3 Newbie   Joined: Mar 2010 Posts: 10 Thanks: 0 Re: math question (cube, cube volume, percent) Actually no cuz I tried that, in fact I tried everything, but I guess I am just not that smart my answers still don't add up to %72.8 which is the answer in the back of the book.
 March 22nd, 2010, 06:12 AM #4 Senior Member   Joined: May 2008 From: York, UK Posts: 1,300 Thanks: 0 Re: math question (cube, cube volume, percent) Consider 3 mutually perpendicular sides of the original cube. Let's say that the length of each side of the cube is $l,$ so that the volume is $l\times l\times l=l^3.$ When we increase the length of a side by 20%, we are adding on an additional 20% or 0.2 of the length. So the new side length is $l+0.2\times l=1.2\times l.$ Can you see what the volume of the cube is now? You can find the percentage increase in the volume via the formula $\text{%}_{increase}=\left(100(\text{vol}_{new}-\text{vol}_{old})\right)\text{%}.$
 March 25th, 2010, 09:53 AM #5 Newbie   Joined: Mar 2010 Posts: 10 Thanks: 0 Re: math question (cube, cube volume, percent) I still don't get it
 March 25th, 2010, 11:34 AM #6 Global Moderator     Joined: Oct 2008 From: London, Ontario, Canada - The Forest City Posts: 7,958 Thanks: 1146 Math Focus: Elementary mathematics and beyond Re: math question (cube, cube volume, percent) (1.2)^3 = 1.728
 March 25th, 2010, 01:30 PM #7 Senior Member   Joined: Mar 2007 Posts: 428 Thanks: 0 Re: math question (cube, cube volume, percent) Longer, but to give more explanation perhaps: Old side = m. Old volume = m * m * m = m^3 New side = (m + 20% of m) = m + 0.2m = 1.2m New volume = (1.2m)(1.2m)(1.2m) = (1.2 * 1.2 * 1.2)m^3 = 1.728m^3 Volume increase = (1.728 - 1)m^3 = 0.728m^3 Percentage increase in volume = (increase / old volume) x 100% = (0.728/1)*100% = 72.8%
 March 25th, 2010, 05:06 PM #8 Newbie   Joined: Mar 2010 Posts: 10 Thanks: 0 Re: math question (cube, cube volume, percent) Wow thanks, I am starting to get it I need more examples to work with but the trouble is the book doesn't seem to have any more questions like this. Could you provide me with some more problems like this if you know any?
March 25th, 2010, 05:40 PM   #9
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Re: math question (cube, cube volume, percent)

Quote:
 Originally Posted by yano could you provide me with some more problems like this if you know any?
Sorry, I don't mind helping, but I'm neither your teacher nor tutor, and don't intend to write the book here. Some things you really need to do for yourself, you know. Try Google, or a used book store, or a library.

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