A boat travels downstream 140 miles and back at its maximum speed. It takes 5 hours to get downstream and 7 hours to get back. Use a system to find the speed of the current.

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5d+7g=140
d+g=current
28+20=40

5d+7g=140
d+g=current
28+20=40
Must be modern math-28 + 20=40??