On a test,Brenda worked 7/30 of an hour, Emma worked 7/20 of an hour and Laura worked 7/15 of an…

On a test,Brenda worked 7/30 of an hour, Emma worked 7/20 of an hour and Laura worked 7/15 of an hour. What is the difference in the times of the girl who worked the longest time and the girl who worked the shortested time?