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There are 60 balls including 5 colours. 7 of them are green.

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There are 60 balls including 5 colours. 7 of them are green. [#permalink]  06 May 2006, 00:30
There are 60 balls including 5 colours. 7 of them are green. The probability that one ball picked out at random is green or yellow is 2/5. At most, what is the number of the yellow balls?
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Re: Probability: Number of yellow balls [#permalink]  11 May 2006, 12:50
getzgetzu wrote:
There are 60 balls including 5 colours. 7 of them are green. The probability that one ball picked out at random is green or yellow is 2/5. At most, what is the number of the yellow balls?

= (2/5) 60 -7 = 24 -7 = 17
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Not sure what it means by "At most, what is the number of yellow balls."

I agree with Prof - there should just be 17 yellow balls.
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P(Green or Yellow)=(G+Y)/60=2/5 => Y=24-G=24-7=17
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There are 60 balls including 5 colours. 7 of them are green. The probability that one ball picked out at random is green or yellow is 2/5. At most, what is the number of the yellow balls?

7 Green balls so 53 balls of other color.

Probability that 1 ball picked is yellow or green = total yellow + green balls / 60 = 2/5

(7+y)/60 = 2/5

Solving for y, y = 17 is the answer.
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