Pens In A Box

pjmcdonnell

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Mar 13, 2019
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Hello, I'm new to the site so sorry if I've done anything wrong😅 I created an account just for this problem because it was bugging me so much. Yes I know it probably has a very simple solution, but it was the last question on one of my GCSE mock exams so I'd like to make sure.

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There are only green pens and blue pens in a box.

There are three more blue pens than green pens in the box.
There are more than 12 pens in the box.

Simon is going to take at random two pens from the box.

The probability that Simon will take two pens of the same colour is 27/55.

Work out the number of green pens in the box.



HELP
 

tkhunny

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Apr 12, 2005
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Rule #1 - If you don't know what it is, give it a name so we can talk about it.

G = # of Green Pens in the Box.

What's next?

Note: This may strike you as an unusual clue: "There are more than 12 pens in the box." This says to me that your solution is likely to lead you to a quadratic equation of some sort. You may get two possible solutions. One will be 12 or less than 12 and the other will be greater than 12. Pick the right one.
 
Last edited:

pka

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Jan 29, 2005
Messages
7,797
Hello, I'm new to the site so sorry if I've done anything wrong😅 I created an account just for this problem because it was bugging me so much. Yes I know it probably has a very simple solution, but it was the last question on one of my GCSE mock exams so I'd like to make sure.
Do you know the binomial notation \(\displaystyle \binom{G+3}{2}=\dfrac{(G+3)(G+2)}{2\cdot 1}\;?\)

If you do then tell us what this is all about: \(\displaystyle \dfrac{\dbinom{G+3}{2}+\dbinom{G}{2}}{\dbinom{2G+3}{2}}=\dfrac{27}{55}\)
 
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