"You may go, but we shall expect you back in time for the noon meal," the Professor told him.
"Give me money," requested the guide.
"Certainly. Let me see, you have worked a week. I gave you five dollars when we started out. You were to have ten dollars a week while you were with us. That leaves five dollars due you," announced the Professor.
"Me work week. Me want ten dollars."
"But, my man, I've already paid you five dollars, which pays you for half of the week. Here is the five dollars for the other half. That's all I owe you. Do you understand?"
"Si señor. But Juan work one week," protested the guide.
"Let me show him," interrupted Tad. He drew ten marks in the sand with a stick, separating them into two groups of five. "Here are ten marks, Juan. We'll call them ten dollars. Understand?"
"Si."
"Well, here are the first five marks in the dirt that the Professor paid you. How many does that leave?"
"Five," gleamed the white teeth.