Since with have both the distance and the velocity of both cars, we can calculate their velocities, which is distance/time.
Roger:
2 miles in 7.5 minutes -> [tex]1h = 60min -> \frac{7.5 min}{60 min} = 0,125h[/tex]
2 miles in 0,125h -> [tex]\frac{2 miles}{0,125h} = 16miles/hour = 16mph[/tex]
Natalie
2 miles in 6.5 minutes -> [tex]1h = 60min -> \frac{6.5min}{60min} = 0,1083h[/tex]
2 miles in 0,1083h -> [tex]\frac{2 miles}{0,1083h} = 18,46 miles/hour = 18,46mph[/tex]
With their velocities in hand you can draw your own conclusions ;)