Time t is in hours. Distance each way d, is also unknown as is t.
RATE TIME DISTANCE
TRIP THERE 180 t+1 d
RETURNING 200 t d
You just made a decimal mistake.
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Another way of showing that you know what you need to do to solve this problem, but as sometimes happens, a computational mistake occurs. Mistake NOT because of lack of understanding - just a slippage in attention.