It is about sequence or mixture

Question

A 20 liters container is full of 80% acid solution.2 liters are drawn off and replaced with water from the mixture; 2 liters are again drawn off and replaced with water. Find the percentage of concentration after 10 replacements

Answer 1

20 liters have 80% acid

after 2 liters are replaced with water the % is 64%

one more similar replacement it's down to .8x64 = 51.2%

do it 10 times and it's down to 80(.8^9) = 10.73741824%

= about 11%