Time-speed problems can be solved easily by graphs.

A cyclists sets off at 6 am travelling at a steady speed of 15 mph.Later at 9 am a second cycllst sets off on the sane journey at a steady speed of 20 mph. When will the two cycllsts meet?

Draw two straight line graphs (time versus distance). Where the two lines intersect will be the solution. Read off the time at that crossing point.