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In the mixture of 126 kg of milk and water, milk and water are in the ratio 5:2 how much water must be added to the mixture to make this ratio 3:2.
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I will not follow your solution and will develop my own solution instead.
In the original mass of 126 kg of the mixture there are 7 (= 5 + 2) equal parts (masses).
Hence, the milk contents is = 5*18 = 90 kg,
and water contents is = 2*18 = 36 kg.
OK.
Next, the final mixture, having 90 kg of milk, must contain = 30*2 = 60 kg ow water to provide the ratio = .
Hence, 60 - 36 = 24 kg of water must be added to the original mixture to satisfy the problem requirements.
Answer. 24 kg of water must be added to the original mixture.
Solved.