# Statistics HELP!!!

• Oct 23rd 2008, 08:50 PM
kim1709
Statistics HELP!!!
I have these three problems I cannot figure out! I will greatly appreciate the help! Thanks in advance!

1. A die is rolled 7 times and the number of twos that come up is tallied. If this experiment is repeated many times, find the standard deviation for the number of twos.

2. In a game, you have 1/44 probability of winning \$138 and a 43/44 probability of losing \$5. What is your expected value?

3. A study of the amount of time it takes a mechanic to rebuild the transmission of a Chevrolet shows that the mean is 8.4 hours and the standard deviation is 1.8 hours. If 40 mechanics are randomly selected, find the probability that their mean rebuild time is less than 8.9 hours.
• Oct 24th 2008, 03:22 AM
mr fantastic
Quote:

Originally Posted by kim1709
I have these three problems I cannot figure out! I will greatly appreciate the help! Thanks in advance!

1. A die is rolled 7 times and the number of twos that come up is tallied. If this experiment is repeated many times, find the standard deviation for the number of twos.

2. In a game, you have 1/44 probability of winning \$138 and a 43/44 probability of losing \$5. What is your expected value?

3. A study of the amount of time it takes a mechanic to rebuild the transmission of a Chevrolet shows that the mean is 8.4 hours and the standard deviation is 1.8 hours. If 40 mechanics are randomly selected, find the probability that their mean rebuild time is less than 8.9 hours.

1. $X_i$ ~ Binomial(n = 7, p = 1/6).

$S_n = X_1 + X_2 + \, .... \, + X_n$.

$Var(S_n) = \, ....$

2. Apply the definition: E(X) = (138)(1/44) + (-5)(43/44) = ....

3. See question 6 of this thread: http://www.mathhelpforum.com/math-he...ssistance.html. The answer, correct to four decimal places, is 0.8541.
• Oct 24th 2008, 06:23 PM
kim1709
Thanks!
Mr. Fantastic!

Thanks so much for your help! You truly are FANTASTIC!!!!

kim1709