Theo left Phoenix traveling 55 mph. Two hours later, Dennis left Phoenix following Theo and he was speeding along at 75 MPH. How many hours will it take Dennis to catch up with Theo?
+486 Dennis gains $77-55=20$ miles on Theo every hour. Theo had a $55*2=110$ hour headstart. So it will take $\frac{110}{20}=\boxed{11/2}$ or $5$ $\frac{1}{2}$ hours for Dennis to catch up to Theo.
