# Question on Normal Distribution

• May 9th 2010, 04:26 PM
asemh
Question on Normal Distribution
Hey,
Im having trouble with this question, i really dont know how to start it...

Calculate the probability that 200 rolls of two dice rolls will unclude

a) more than 30 sums of 5
b) between 30 and 40, inclusive, sums of 5

• May 9th 2010, 04:46 PM
pickslides
Are you sure this is normal? I am thinking it is a normal approximation to binomial.

Anyhow, first find the probability of the die summing to 5 on any given roll.

What do you get?
• May 9th 2010, 05:23 PM
asemh
Yea sorry i meant Normal approximation =\

ooh nvm i got it

This is what i did.. found that the probability of getting 5 from two dice is 1/9

then i found the mean (1/9 * 200) = 22.22
standard deviation = 4.44

and then i did P(x>30.5)
and i got the right answer which is 0.0312

Well maybe try something like $P(X\geq 30 ) = P\left(Z\geq \frac{30 - np}{\sqrt{np(1-p)}}\right)$
Now can you find $n$ and $p$ ??