Question

How much pure alcohol should be added to $400ml$ of strength $15\%$ to make its strength $32\%$?

• A. $50ml$
• B. $75ml$
• C. $100ml$
• D. $150ml$

Answer 1

Answer: (C)

$100ml$

The amount of alcohol in $400ml$ solution of $15\%$ strength

$=400\times \frac{15}{100}ml=60ml$

Let the required amount of alcohol to be added $=x$ ml.

It will increase the amount of the solution by $x$ ml.

$\therefore$ The amount of alcohol now $=(60+x)ml$

and the amount of the solution$=(400+x)ml$.

Then, the strength $=32\%=\frac{60+x}{400+x}=\frac{32}{100}$

$\Rightarrow 25x+1500=3200+8x\Rightarrow 17x=1700\Rightarrow x=100$.

So, the required amount of pure alcohol to be added $=100ml$.