Math Is Fun Forum

mathster

Member

Registered: 2015-10-09

Posts: 15

Quick problem.

Alana is making a ball out of rubber bands. When the ball has 54 rubber bands, it has a diameter of 3 cm. How many rubber bands should Alana add to the ball to increase its diameter by 1 cm? Assume that all of Alana's rubber bands have the same volume.

I am getting 74. Could anyone please tell me if that's correct? Thanks.

(with 54 rubber bands).

(with new amount of rubber bands).

Is my solution correct?

David

Member

From: Bumpkinland

Registered: 2014-04-23

Posts: 3,164

Re: Quick problem.

Hi;

I could be wrong, wait for an expert.
[WRONG

David.

Last edited by David (2015-10-16 01:55:33)

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His dominion is an everlasting dominion, Which shall not pass away, And His kingdom the one Which shall not be destroyed.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,621

Re: Quick problem.

hi mathster

I also get 74 bands by the same method. 

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

David

Member

From: Bumpkinland

Registered: 2014-04-23

Posts: 3,164

Re: Quick problem.

I might need some explanation.

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His dominion is an everlasting dominion, Which shall not pass away, And His kingdom the one Which shall not be destroyed.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,621

Re: Quick problem.

hi David

The diameter at first is 3cm so the radius is 3/2 cm.  So you can work out the volume and divide by 54 to get the volume of a single band.

Now compute the increase in volume (with a new radius of 2):

Now divide this by the volume of a band to get how many more you will need:

Hope that helps,

Bob

ps.  I didn't look at how you got your answer.  Maybe it was mostly right.    If you re-post it I'll see if I can see what went wrong.  Don't worry about posting something that may be wrong.  It happens to me a lot. 

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

David

Member

From: Bumpkinland

Registered: 2014-04-23

Posts: 3,164

Re: Quick problem.

I see what is wrong now... I squared the radius... LOL! Thanks!

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His dominion is an everlasting dominion, Which shall not pass away, And His kingdom the one Which shall not be destroyed.