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In the summer, water consumption is known to decrease overall by 25%. A Water Board official states that in the summer household consumption decreases by 20%, while other consumption increases by 70%.

Which of the following statements is correct?

  • A.The ratio of household to other consumption is 8/17
  • B.The ratio of household to other consumption is 1/17
  • C.The ratio of household to other consumption is 17/8
  • D.There are errors in the official's statement.

Answer: D

The correct statement is that there are errors in the official's statement.

Let household consumption be h and other consumption o before summer. The official's figures, together with the known overall fall of 25%, require

\[ \begin{aligned} 0.8h + 1.7o &= 0.75(h + o) \\ 0.05h + 0.95o &= 0 \end{aligned} \]

Both h and o are positive, so the left side is positive and can never be zero. No positive amounts of household and other consumption can fit: a 20% fall in one part and a 70% rise in the other can never add up to an overall fall of 25%, since even the fall in the first part is only 20%.

The three ratio options therefore describe a situation that cannot exist; none of them makes the equation hold.

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