It is knowledge as well as interview questions. What would you do if it were you?

First buy 18 bottles of drinks, then you can exchange 6 bottles of drinks, or you can return 2 bottles of drinks, borrow 1 bottle from the merchant, and exchange the last 2 bottles for 1 bottle and return them to the merchant.

So it is 18+6+2+ 1+=27.

First, divide the eight balls into two piles and put them at both ends of the balance. The upward side proves to be broken.

Divide the four into two piles (the upward side of the balance) and put them on the balance. Do you have a light color? A broken one

Finally, divide the two into two piles (the side with the balance up last time), and then weigh them on the balance, and the light side will be broken.

See the figure for details of question 3.