Ratio Mr Koh : Mr Sim = 7 : 2 at first. Mr Sim had $100.

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• Mr Koh at first = \(\frac{7}{2}\) × $100 = $350
• Let Mr Koh give $275 to Mr Sim
• Mr Koh after = 350 − 275 = $75
• Mr Sim after = 100 + 275 = $375
• Ratio after = 75 : 375 = 1 : 5
• Mr Koh has \(\frac{1}{5}\) as much as Mr Sim

Mr Koh gave $275 to Mr Sim.
Check: Starting total $450 = ending total $450. ✓