Calculate the confidence level with z-score and alpha unknown?

A management engineer at Country Squire Hospital determined that she needs to make a work sampling study to see whether the proportion of idle time in the diagnostic imaging department had changed since being measured in a previous study several years ago. At that time, the percentage of idle time was 8 percent. If the engineer can only take a sample of 850 observations due to cost factors, and can tolerate an allowable error of 0.02, what percent confidence level can be obtained from the study?