I can count

At the MPH cashier counter.

Me: I understand that purchase made above RM100 entitles me to a mug?
Cashier: Yes.
M: My maths very 'teruk' lar. (suck). Can you help me count if these are above Rm100

Place books on the counter.
Cashier punches her machine.

C:Yes, the total is Rm114.++
M:Great

Made payment, walked away to the gift counter to collect my mug. Noticed something amiss in my receipt. Walks back to the cashier.

M:Excuse me, isn't this book on a 30%? You gave me 20%.
C: *Embarassed* Oops, sorry.

Got a another supposedly senior cashier to solve the problem.
Senior cashier punches the calculator. She tells me that MPH currently owes me Rm1.7+

M: That can't be right, it should be Rm3.59.

Senior cashier punches the machine again, looking agitatedly at the receipt, books, the earlier cashier and later ME.
Explains incoherently to me how she managed to come up with that number.

C: It still doesn't make sense. You owe me 10%. How can 10% be only Rm1.7+?

By then, the previous customers who were in line (who heard me proclaiming that I'm teruk in maths) were amused at my 'blurness'.

The senior cashier was so confident with her calculations, that I myself begin to doubt my own calculations.
Fortunately, the earlier cashier who had started this mess, had the conscience to come into the picture to clarify.
Finally, I understood. The senior, had assumed that the mistake was 25%, not 20%.
She didn't seem apologetic at all. Nobody was.

My only regret was, all the customers in line who were amused, weren't around anymore to see that "I" was RIGHT. Me, Mua!

Sigh... Can anybody recommend any Anger Management class to me?
Please?