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Answer:
6,000 meters
Step-by-step explanation:
Length of one lap d = 400 m
Lee's speed = u = 7.2 km/h. Shelby's speed v = 9 km/h
After time t, Shelby will run x meters to cover exactly 3 more laps than Lee.
So x = vt
and
x - 3d = ut
Note t = x/v, then
x - 3d = x*u/v
x (1 - u/v) = 3d
x = 3d/(1 - u/v) = 3*400/(1 - 7.2/9) = 6,000 meters