And it was different, as she discussed with Mr. J.L.B. Matekoni when, an hour or so later, they had their own rather delayed dinner. He said that he was not at all sure if schools taught mental arithmetic any more. “Take Charlie,” he said, referring to the older of his two apprentices. “If you ask him to do some simple calculation—such as what the capacity of a fuel tank might be if you take a bit of it off—he looks blank and reaches for his pocket calculator. We can do those things in our head, can’t we, Mma?”

Mma Ramotswe thought about this. It was true that she could work out how much change was due at the supermarket till, but when it came to fuel tanks and their capacity, she was not so confident. But the general point that Mr. J.L.B. Matekoni was making certainly stood: there were some things that just had to be learned through effort, and she was not sure how popular effort currently was. “It is all different, Rra,” she said. “The world is all different. But people like Charlie can do other things, you know. These people who cannot add up can do other things very well.”

Mr. J.L.B. Matekoni looked doubtful. “I’m keen to hear what those things are,” he said.

“They are good at …,” began Mma Ramotswe, quickly searching her mind. “They are good at computers and things like that.”

“Maybe,” he said. “But there are things other than computers, Mma. There are proper machines with cogs and grease. Are they good at those? Are they good at fixing ploughs?”

The mention of ploughs reminded Mma Ramotswe of Mma Potokwane, who had recently asked Mr. J.L.B. Matekoni to fix the small plough that they used to till the fields at the orphan farm. He had done it, of course, as he always did, and charged no fee, again as he always did. Now she told him about the visit the matron had paid to the No. 1 Ladies’ Detective Agency that morning.

“She came in an unknown car,” she said. “She told us that it had been given to the orphan farm by a big donor. Mr. Ditso Ditso.”

“Ow!” said Mr. J.L.B. Matekoni. “That man is very big.”

“Yes. But the car is only the beginning. He has said that he will give a whole lot more in future.”

“He is very kind as well.”

“Yes,” agreed Mma Ramotswe. “But the trouble is, Mma Potokwane doesn’t like a new scheme he has cooked up.”

Mr. J.L.B. Matekoni listened gravely as Mma Ramotswe told him about the reservations that Mma Potokwane had expressed as to the effect that the proposed changes at the orphan farm would have. “And then,” she went on, “Mma Potokwane wondered about whether his money was honestly acquired. Do you think it is, Rra?”