Question

A 60-litres solution of alcohol and water has 20% water. How many litres of alcohol must be added to the solution to make the water content 15%?

• A. 10 litres
• B. 20 litres
• C. 30 litres
• D. 40 litres

Answer 1

Answer:

B. 20 litres

Given:

• Total solution = 60 litres

• Water = 20% of 60 = 12 litres

• Alcohol = 48 litres

Let x litres of alcohol be added.

Then:

• Water remains 12 litres

• New total volume = 60 + x

We want water to be 15% of the new solution:

$12 = 15\% \times (60+x)$
$12 = 0.15(60+x)$
$12 = 9 + 0.15x$
$3 = 0.15x$
$x = \frac{3}{0.15} = 20$

Answer: 20 litres of alcohol must be added.