One person bought milk from the vendor.
He asked the vendor for the price.
The vendor said: NT$3 per bottle ,NT$10 three bottles.
The man bought one bottle for NT$3 and repeated three times.
The man later proudly said to the vendor:”Haha, I bought 3 bottles only for NT$9.”
The vendor said: “Haha, since I did this, I can sell 3 bottles at a time.”