Undercalculation of each bottle: 9-6=3 yuan, * * * Undercalculation: 3*6= 18 yuan.

The actual undercharged money can be bought: 18/9=2 bottles.

You can actually buy 6-2=4 bottles.

Solution 2:

For example, the ratio of unit price to original unit price is 6/9=2/3.

In the case of constant total price, the quantity is inversely proportional to the unit price, and the original quantity is regarded as 2/3 of the quantity.

Actual quantity: 6*(2/3)=4 bottles.