Well, darling, 850 to the nearest 1000 is still 1000. It's like saying you're almost a millionaire when you've only got a thousand bucks in your bank account. Close, but not quite there yet. Keep reaching for the stars!
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There are 1000 millimetres in one metre. Therefore, 850 millimetres is equal to 850/1000 = 0.85 metres.