Find sample size suggested by statement of error range.

[serenity26]

A Philadelphia article about poll results states, "In theory, in 18 cases out of 20, the results from such a poll should differ by no more than two percentage point in either direction from what would have been obtained by interviewing all voters in the United States." Find the sample size suggested by this statement.

i have no idea formula to use. and my book gives no example please help

 

[royhaas]

Your book should give at least one formula for calculating the sample size for a binomial proportion, probably in the same chapter that deals with a confidence interval for a proportion.