Since Paula left home an hour ago driving at 40mph, it means she has already gone 40 miles by the time Dan left. Dan is driving at 50mph, which is 10 greater than 40. This means that every hour, Dan will drive 10 more miles than Paula, lessening their difference by 10. 40 divided by 10 is 4, so iit will take Dan 4 hours to catch up to Paula:)
