Question 184212
the total distance is 20 mi, so r+w=20 ... and w=20-r


the total time is 3 hr ... since rate times time equals distance, time equals distance divided by rate


(r/7)+(w/3)=3


substituting ... (r/7)+((20-r)/3)=3


subtracting 20/3 ... (r/7)-(r/3)=-11/3


multiplying by 21 (LCD) ... 3r-7r=-77 ... -4r=-77


dividing by -4 ... r=77/4


since time equals distance divided by rate, Mark's running time is (77/4)/7 or 11/4 hr (2hr45min)