Since the plane travels 213 miles per hour, it must travel on average 1 mile every 1/213rd of an hour.We know that there are 60 minutes in 1 hour, and 60 seconds in a minute, so there are 60 x 60 = 3600 seconds in an hour.Thus 1/213 hours is equivalent to 3600/213 seconds (as a fraction) or 16.9014 seconds.Thus to 1 d.p. our estimate is 16.9 seconds.