Can you explain why the answer to this question is 6.416?

A major airline 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 airplanes have arrived late. In a random sample of 1,240 airplanes, 310 airplanes have arrived late. If we are conducting a hypothesis test of a single proportion to determine if the proportion of late arrivals has increased. That is, the alternative hypothesis states that the late arrivals have increased compared to the historical proportion. What is the value of the calculated test statistic?