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A chemist mixes two liquids $1$ and $2$. One litre of liquid $1$ weighs $1$ kg and one litre of liquid $2$ weighs $800$ gm. If half litre of the mixture weighs $480$ gm, then the percentage of liquid $1$ in the mixture, in terms of volume, is

1. $85$
2. $70$
3. $75$
4. $80$

$\textbf{First Method:}$

Given that,

• One litre of liquid $1$ weighs $= 1 \; \text{kg} = 1000 \; \text {gm}.$
• One litre of liquid $2$ weighs $= 800 \; \text{gm}.$
• And half litre of mixture weighs $= 480 \; \text{gm}.$

When both liquids given and mixture also given, we can use the alligation method.

Half litre of mixture weighs is $480 \; \text{gm}.$

Therefore, the weighs of liquid $1$ and liquid $2$ also become half.

Now, we can apply the alligation method. $\therefore$ Percentage of liquid $1$ in the mixture $= \frac{4}{5} \times 100 = 80\%.$

$\textbf{Second Method}:$

$1$ litre mixture weigh is $480 \times 2 = 960 \text{gm}.$ $\therefore$ Percentage of liquid $1$ in the mixture $= \frac{4}{5} \times 100 = 80\%.$

Correct Answer $: \text{D}$

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