After Mr Tan transfers some money to Mrs Tan, the total stays constant at 9 units.

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• Final ratio: Mr Tan : Mrs Tan = \(1 : 8\)
• Total = \(1 + 8 = 9\) units
• Mrs Tan received the transfer and ended with 8 units
• Mr Tan kept 1 unit after giving away the rest

Final Answer: 1 : 8