question: Colorado City uses 40% of the water from the Colorado River. If 80% of that water is used for industrial purposes, what percent of the river's total water is used by the city for non-industrial purposes?
answer: First find the percentage of water used for non-industrial purposes: 100% - 80% = 20%
  Then multiply that percentage by the percentage of water taken from the river: 20% * 40% = 8%
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