# Probability of people having answering machines

1. Jun 6, 2009

### Larrytsai

According to a research company survey, 70% of telephone subscrivers have an answering machine. If a telemarketing campaign contacts 200 telephone subscribers, what is the probability that 150 or more iwll have an answering machine?

Im lost for the tree diagram but and i have no clue for what method but my attempt was 0.7x 200 = 140 then 140/150, 1-(140/150)

2. Jun 6, 2009

### CompuChip

Re: Probabilitiy

Have you learned about binomial distribution yet?
Then can you argue that the number of subscribers that has an answering machine is binomially distributed?

3. Jun 6, 2009

### HallsofIvy

Staff Emeritus
Re: Probabilitiy

Of 200 people, 70% of whom, have answering machines, what is the "expected value" (mean) of the number who have answering machines? What is the standard deviation?

And if the number, 200, makes the calculations too tedious, you could use the "normal distribution approximation"- the normal distribution having the same mean and standard deviation. Don't forget to use the "integer correction". Since the normal distribution is a continuous distribution while the binomial is discrete, 149.5 would be rounded to 150.