Geometric distribution:A teacher provides pizza for his clas

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wind

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Sep 20, 2006
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A teacher provides pizza for his class if they earn an A-average on any test. The probability of the class getting an A-average on one of his tests is 8%. What is the probability that the class will not earn a pizza for the first 7 tests?

\(\displaystyle \L\ P(x)=q^{x}p\)

so if x is the number of failures and falures would be getting an an A average, then x=7 right? so

\(\displaystyle \L\ P(x)=\frac{92}{100}^{7}\frac{8}{100}\)

\(\displaystyle \L\
P(x)=0.0446277281\)

...but that answer is wrong

can someone tell me what I did wrong?


Thanks
 
Re: Geometric distribution:A teacher provides pizza for his

wind said:
A teacher provides pizza for his class if they earn an A-average on any test. The probability of the class getting an A-average on one of his tests is 8%. What is the probability that the class will not earn a pizza for the first 7 tests?

\(\displaystyle \L\ P(x)=q^{x}p\)

so if x is the number of failures and falures would be getting an an A average, then x=7 right? so

\(\displaystyle \L\ P(x)=\frac{92}{100}^{7}\frac{8}{100}\)

\(\displaystyle \L\
P(x)=0.0446277281\)

...but that answer is wrong

can someone tell me what I did wrong?


Thanks
Click to expand...
You've given the probability the first A-average occurs on the 8th test. (That's what the geometric distribution is for.) But the question only says the first 7 tests will not have an A-average. It says nothing about the 8th test. So leave the 8th test out of your probability.
 
Thanks JakeD!

It says nothing about the 8th test. So leave the 8th test out of your probability.
Click to expand...

so would it be

\(\displaystyle \L\ P(x)=\frac{92}{100}^{7}\frac{8}{100}^{0}\)
 
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