Question

A solution of 8% boric acid is to be diluted by adding a 2% boric acid solution to it. The resulting mixture is to be more than 4% but less than 6% boric acid. If we have 640 litres of the 8% solution, how many litres of the 2% solution will have to be added?

Answer 1

Let's add $x$ liter of $2\%$ boric acid solution.

Let's find $x$ when final solution is $4\%$ boric acid

Equating water content

$0.98x+.92\times 640=.96(x+640).02x=25.6x=1280$

Similarly for $6\%$ boric solution

$0.98x+.92\times 640=.94(x+640).04x=12.8x=320$