A psychologist finds that fidgety patients tap their fingers on average 500 times during a 2 hour period. You wish to test this claim for the same patients, but in a room decorated with soothing colors and soft lighting. You believe that a warmly decorated room may alter the fidgeting behavior, causing subjects to tap their fingers less than they do under normal conditions. A random sample of 400 patients has a mean of 420 taps and a standard error of 30 taps in a 2 hour period.

Find the margin of error for the average tapping of fingers during a 2 hour period.

Margin of error =___________