First, after drinking 24 bottles of water, there are 24 empty bottles, and you can change 8 bottles of water.

Drink 8 bottles of water, leaving 8 empty bottles. Replace six empty bottles with two bottles of water. If you drink any more, there will always be four empty bottles.

Three more bottles were replaced with 1 bottle of water, and only two empty bottles were left after drinking, and then they could not be replaced.

So they drank 24+8+2+ 1=35 bottles.