# Probability GCSE

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• Nov 14th 2010, 01:21 PM
Caladbolg
Probability GCSE
Hello everyone, this is probably really easy. I'm revising for my Maths exam tomorrow on data handling and I have this question that I can't seem to get the hang of

"A cube is drawn at random and replaced 250 times, how many times would you expect a red cube to have been drawn?"

The probability for picking a red cube is apparently 0.35

I assumed it was roughly a quarter so I tried doing 250 / 3 and still not the answer I was looking for.

Can someone help me understand this?

thanks a lot.
• Nov 14th 2010, 02:29 PM
Plato
The way in which you have posted the question makes it a bit vague.
It appears that this binomial.
The success is getting a red cube with a probability of 0.35 in 250 trials.
The expected value for a binomial RV is $\displaystyle p\cdot N$.
Here $\displaystyle p=0.35~\&~N=250$.

Warning: My reading of what you meant may be wildly off.
• Nov 14th 2010, 02:35 PM
Caladbolg
Yeah I know, it's how it's written on my practice sheet. I don't think it's that, it's just GCSE data handling probability.

Red - 0.35, Blue - 0.15, Green - 0.2, Black 0.3

A cube is drawn at random and replaced 250 times (picked up and put back)

How many times would you expect a red cube to have been drawn?

That's all it asks, I had trouble trying to work this out, I was told the answer was 88? but I want to understand how to work it out
• Nov 14th 2010, 02:40 PM
Plato
Quote:

Originally Posted by Caladbolg
I was told the answer was 88? but I want to understand how to work it out

Well $\displaystyle (0.35)(250)=?$
• Nov 14th 2010, 02:41 PM
Caladbolg
oh I see, 0.35 x 250 gives you 87.5 and you round it up? or is 87.5 theee answer?