By: Al Jupri, M.Sc.

To start his mathematics lessons, Mr. Yudi always associates with everyday events.

“Kids, today we’re going to continue math lessons. Last night, you bought martabak at a passing seller in front of the house. If you have money Rp. 15,000, and the price of martabak is Rp. 8000, how much money did the father receive, the children?”

For a moment of silent class, all the students seemed to think.

**Udin** : “Rp. 7000, sir!”

**Jerry:** “Yes, sir, Rp. 7.000.”

The other students seem to agree and answer Udin and Jerry.’ But, Tom disagrees.

**Tom:** “No, sir! Obviously, the return is Rp. 2000.”

“How the hell are you Tom, you’re wrong! Kan Rp. 15.000 – Rp. 8000 = Rp. 7000” said Udin who seemed approved by Jerry.

**Mr. Yudi:** “Okay, Tom. Why is your answer Rp. 2000?”

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Well, dear readers, why is Tom’s answer so?

**Note:** Although the problems raised in this article are very simple, do not be underestimated. Please explore the mathematical and didactic values contained in the proposed issue!