You have two large 2 liter container and a small cup containing 0.1 liter. At the beginning you have 1 liter of 10% acid solution in the left container, and 1 liter of 90% acid solution in the right one. One step consists of taking one cup of solution from the left container and putting it into the right one, and after that taking one cup of the solution from the right container and putting into the left container. How much of these steps are needed until the difference of the acid concentrations of both containers is less than 5%?
Erich Prisner, August 2003