To strengthen 400 ml of 15\% alcohol solution to 32\% alcohol solution, a pharmacist would need additional pure alcohol amounting to
(a)50 ml
(b)75 ml
(c)100 ml
(d)125 ml
Answer
Answer (as printed): C
Explanation
Quantity of alcohol in 400 ml solution = 15\% of 400 ml $=60 \mathrm{ml}$. Let $x \mathrm{ml}$ of alcohol be added. Then, $\frac{60+x}{400+x}=\frac{32}{100} \Rightarrow 6000+100 x=12800+32 x$ $$\Rightarrow \quad 68 x=6800 \Rightarrow x=100 .$$
Explanation as extracted from the printed page; notation may be imperfect.