# How much water must be added to 20 L of a 10% solution of boric acid to reduce it to a 4% solution?

Aug 13, 2015

You have to add 30 litres of water

#### Explanation:

At the beginning you have 20 L of 10% solution, so it consists of 2L of acid and 18 L of water.

Now you have to calculate the amount of water to add to change the solution to 4%.
To do this you have to solve equation:

$\frac{2}{20 + x} = \frac{4}{100}$

The equation comes from the fact that the amount of acid is unchanged (2L) and the sum of water and acid is increased by $x$ (added water)

$\frac{2}{20 + x} = \frac{2}{50}$

$40 + 2 x = 100$

$2 x = 100 - 40$
$2 x = 60$
$x = 30$