“It says here,” said Mma Ramotswe patiently as she sat with Puso at the table that Mr. J.L.B. Matekoni had made for the boy’s bedroom, “that it takes one man one hour to dig a ditch.”

Puso looked up at her. “That is very quick, Mma. One hour? Could anybody dig a ditch in one hour?”

“It’s just for this sum,” said Mma Ramotswe. “And we shouldn’t worry about that just now. What else does it say?” She looked at the crumpled sheet of paper on which the homework exercises had been printed out. “It says that if it takes one man one hour to dig the ditch, then how long would it take for three men to dig the same ditch? What do you think is the answer to that, Puso?”

Puso frowned. “It would be very hard for three men to dig one ditch, Mma. They would always be getting in each other’s way. So it would probably take longer than it would take one man to do it. Maybe two hours?”

Mma Ramotswe smiled. “We don’t have to worry about practical things when we’re doing sums,” she said. “You can forget about things like that. What you must do is divide one by three. That will tell you.” She paused. She was not sure whether he had learned about fractions yet, and the trouble, anyway, was that everything was different these days. Children did not learn to count in the same way as they used to. And she was not even sure whether fractions had been abolished altogether.

“Let’s think of it in minutes,” she said at last. “One hour is sixty minutes. So if you divide sixty by three, what do you get? That is how long it would take three men to do the work done by one.”

Puso closed his eyes. Opening them, he pursed his lips with effort before he replied, “Ten minutes? No, maybe five.”

Mma Ramotswe shook her head. “No, that’s not right. The answer is twenty minutes, Puso. And do you know how I got that answer?”

The boy shrugged. “They could do it much more quickly if they had somebody like Mma Potokwane ordering them about. She’d make them dig faster, wouldn’t she, Mma?”

She did not reply to this, even if what Puso said was quite true. There was nobody like Mma Potokwane to get things done, and that applied, she suspected, as much to ditches as it did to anything else.

“I think you should try to do this by yourself,” she said. “If you can’t do these sums, then ask the teacher to help you. I am not sure how you do these things these days—it is all rather different.”