After hunting by European settlers, the bison population of 60 million was reduced to _______ of its original size.

The original population was about 60 million, and hunting reduced it to roughly 1,000 bison. The remaining fraction is 1,000 ÷ 60,000,000 = 1/60,000, or 1 : 60,000. So the population was reduced to 1 : 60,000 of its original size.

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