Explanation

If Sue completed exactly one more lap than Rob, she ran 10 more miles than Rob. If Rob ran d miles, then Sue ran d + 10 miles. Rob and Sue began running at the same time, so they ran for the same amount of time. Let t represent the time they spent running. Fill out a chart for Rob and Sue, using the formula Distance = Rate × Time (D = RT):

	D(miles)	=	R(miles per hour)	\(\times\)	T (hours)
Rob	d	=	6	\(\times\)	t
Sue	d+10	=	8	\(\times\)	t

There are two equations:

\(d = 6t d + 10 = 8t\)

Substitute 6t for d in the second equation and then solve for t:

\(6t + 10 = 8t\)

\(10 = 2t\)

\(5 = t\)

Hence option C is correct!
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Difference in distance/Difference in speeds = 10/(8-6) = 5

8t - 6t = 10
=> 2t = 5