Ruby is moving from Minneapolis to Chicago, which is 400 miles away. She is traveling at an average speed of 50 miles per hour. Ruby stops for gas after a couple of hours and wonders how much further

Ruby is moving from Minneapolis to Chicago, which is 400 miles away.  She is traveling at an average speed of 50 miles per hour. Ruby stops for gas after a couple of hours and wonders how much further she needs to go.If  represents distance and  represents time, which linear equation would indicate the amount of miles Ruby still has to travel to Chicago?