The quantity of water (in ml) needed to reduce 9 ml shaving lotion containing 50\% alcohol to a lotion containing 30\% alcohol, is
(a)4
(b)5
(c)6
(d)7
Answer
Answer (as printed): C
Explanation
Quantity of alcohol in 9 ml lotion $=\left(\frac{50}{100} \times 9\right) \mathrm{ml}=4.5 \mathrm{ml}$. Let the water to be added be $x \mathrm{ml}$. Then, $\frac{4.5}{9+x}=\frac{30}{100} \Leftrightarrow 270+30 x=450 \Leftrightarrow x=6 \mathrm{ml}$.
Explanation as extracted from the printed page; notation may be imperfect.