Question

18 litres of mixture of acid and water contains 30% acid .how many litres of water should be withdrawal to make acid 90% .

Answer 1

Amount of acid in mixture is
$18 \times \frac{30}{100} = 5.4 \: liter$
Let x be the amount of water to be withdrawn.
Then,
$\frac{5.4}{18 - x} \times 100 = 90 \\ \frac{5.4}{18 - x} = 0.9 \\ 5.4 = 16.2 - 0.9x \\ 0.9x = 10.8 \\ x = 12 \: liter$