Multiple choice

A tap supplies water at 10oC and another tap at 100oC. How much hot water must be taken so that we get 20 kg of water at 35oC?

  1. 5.56 kg

  2. 8.18 kg

  3. 12.5 kg

  4. 45 kg

  5. 4.54 kg
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A Correct answer
Explanation

 Let mass of hot water be m kg. Mass of cold water = (20 - m)kg Heat taken by cold water               = (20 - m) x 1 x (35 – 10) Heat given by hot water              = m x 1 x (100 – 35) Law of mixture gives Heat given by hot water = Heat taken by cold water m x 1 x (100 – 35) = (20 - m) x 1 x (35 – 10) 65 m = (20 - m) x 25 65 m = 500 – 25 m 90 m = 500      m = 500/90         = 5.56 kg

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