Let \(x\) litres be poured from the first vessel (pure alcohol) into the second vessel.
- Second vessel: \(x\) litres alcohol + \((20 - x)\) litres water.
This mixture is poured back into the first vessel until it is full: the amount poured back = \(x\) litres of mixture (alcohol fraction in it = \(\frac{x}{20}\)).
After this exchange, when \(6\frac{2}{3}\) litres = \(20/3\) litres is transferred from the first to the second, they have equal alcohol.
Using concentration balancing equations, solving yields \(x = 10\) litres.
Thus the original transfer was \({10 \, \text{litres}}\).