What percentage

Petra made lemonade from one liter of 100% fruit juice and three liters of water. When she tasted it, she found that it was too sweet and would not be tasty for everyone. She left two liters of it for her friends. She added two more liters of water to the remaining two liters.
What percentage of the drink did her friends drink and what percentage did the others drink?

Correct answer:

p₁ =  25 %
p₂ =  12.5 %

Step-by-step explanation:

$$\begin{gathered} S=1\text{l} \\ V=3\text{l} \\ {p}_1=100\cdot \frac{S}{S+V}=100\cdot \frac{1}{1+3}=25\% \end{gathered}$$

$$\begin{gathered} V_2=2\text{l} \\ {p}_2=100\cdot \frac{S/2}{(S+V)/2+V_2}=100\cdot \frac{1/2}{(1+3)/2+2}=\frac{25}{2}\%=12.5\% \end{gathered}$$

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