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SPEED TIME DISTANCE
A r t 6300
B r+50 t-3 4500
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,... three hours less time.
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SPEED TIME DISTANCE
A r 6300/r 6300
B r+50 4500/(r+50) 4500
Difference 3
Now only one variable is needed, that of either plane, here choosing r for plane A.

Solve for r.