How many litres of water should be added to a 30 litre mixture of milk and water containing milk and water in the ratio of 7 : 3 such that the resultant mixture has 40% water in it?

A. 5 B. 2 C. 3 D. 8 Answer: Option A
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Solution(By Apex Team)

Total quantity of mixture $\begin{array}{l} =\frac{7}{7+3} \times 30 \text { litres }\\ =21 \text { litres } \end{array}$ And quantity of water in the mixture $\begin{array}{l} =\frac{3}{7+3} \times 30 \text { litres }\\ =9 \text { litres } \end{array}$ Let water to be mixed ‘a’ litre Then, $\begin{aligned}&\Rightarrow(30+a)\times\frac{40}{100}=9+a\\ &\Rightarrow120+4a=90+10a\\ &\Rightarrow120-90=10a-4a\\ &\Rightarrow30=6a\\ &\Rightarrow a=5\end{aligned}$ Hence, 5 litres water mixed in the mixture.

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