Stan can drive from Atlas to Billings at a constant rate in

Stan can drive from Atlas to Billings at a constant rate in four hours. If he increases his rate by 15 miles per hour, he could complete the trip in three hours. Find the number of miles between Atlas and Billings.

D=RT
D=R*4
D=(R+15)3
4R=3R+45
4R-3R=45
R=45 MPH IS THE RATE FOR THE 4 HOUR TRIP.
D=45*4=180 MILES IS THE DISTANCE BETWEEN ATLAS & BILLINGS.
D=(45+15)3
D=60*3
D=180 DITTO.