How come it’s not 30

My two friends and I are eating ice cream.
So we decide at the end that we should share.
The seller tells us 30 Baht, it seems.
So 10 Baht each, and it’s fair.

But when I give a seller 30 Baht, 
he tells me he gives us a 5-Baht discount.
So I accept his offer very fast,
because we can save some amount.

Then I give one Baht back to each friend including me
So each will eventually pay nine Baht right?
Then I will have two Bath left, you must see.
The problem is not that we are going to fight.

What is confusing is that 9 times 3 is twenty seven.
Plus two Baht left in my hand, it is just twenty nine.
Where is one Baht gone? Why on earth this happens?
It must be thirty, I dare you to try.

This is just a math problem I give my children,
At the first, when they tried to solve they were just confused.
After spending sometime eventually at the end……
they were simply very amused.