A drum contains 80 litres of a 90% pure acid solution. If 8 litres of this solution are removed and replaced with 8 litres of pure water, what is the new percentage of acid in the solution?

Solution

Initial acid \(=80\times\frac{90}{100}=72\) litres. Acid removed \(=8\times\frac{90}{100}=7.2\) litres. Remaining acid \(=72-7.2=64.8\) litres. Adding pure water does not change the quantity of acid, and the total volume remains 80 litres. New acid percentage \(=\frac{64.8}{80}\times100=81\%\). Trick: The removed solution contains the same 90% acid concentration as the original mixture.