Question 815721
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Hi,
80% of the students surveyed indicated that their parents pay for their mobile phone expenses. 20 students selected at random. 
Binomial Probability Distribution Values: 
Note: The probability of x successes in n trials is: 
P = nCx* {{{p^x*q^(n-x)}}} where p and q are the probabilities of success and failure respectively. 
In this case p = .8 & q = .2 and n = 20  nCx = {{{n!/(x!(n-x)!)}}}

(i) P(x=20) = (.8)^20(.2)^0 = .0115  0r 1.15%
(ii) At least 8 have their parents pay for their mobile phone expenses?
P(x&#8805;8)= 1- P(x&#8804;7)**  TI Calculator:  1 - binomcdf(20,.80,7)= .999998
excel function : 1 - BINOMDIST(7,20,0.8,TRUE)
(iii) No more than 5 students pay for their own mobile phone expenses? 
         TI Calculator:  binomcdf(20,.80,5) = .00000018
excel function : BINOMDIST(5,20,0.8,TRUE)
If You do not have access to a TI Calculator or an Excel Worksheet,
might recommend STATTREK.com as a reference.