statistics
posted by julie .
A major taxi company is concerned that its proportion of late arrivals has substantially increased in the past month. Historical data shows that on the average 18% of the company taxis have arrived late. In a random sample of 1,240 taxis, 310 taxis have arrived late. If we are conducting a hypothesis test of a single proportion to determine if the proportion of late arrivals has increased: What is the value of the calculated test statistic?

Using a formula for a binomial proportion onesample ztest with your data included, we have:
z = .25  .18 >test value (310/1240 = .25) minus population value (.18)
divided by
√[(.18)(.82)/1240] >.18 is test value, .82 is 1  .18, and 1240 is the sample size
Finish the calculation for the value of the test statistic. 
6.416