Well, isn't that a happy little question! 850 is closest to 1000 because it's only 150 away. Just imagine a beautiful landscape with 850 trees, and you're almost at 1000 trees - you're almost there, just a few more steps to go!
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There are 1000 millimetres in one metre. Therefore, 850 millimetres is equal to 850/1000 = 0.85 metres.