The "post-office" at Chitengo was rather touching too. It was a tiny hut, presided over by a huge African with the stupidest face that I have ever seen. We looked at him in dismay, for our wants were fairly complicated - airmail stamps for postcards to Johannesburg, France, England and Scotland, letters to England, sea-mail stamps for the postcards that we were too mean to send airmail, and the whole issue further complicated by Aunt Sophie who wanted to send an airmail postcard to a friend in Australia. None of us had the faintest idea of how many escudos would be needed - we had barely mastered how many escudos there were to a pound. But the postmaster, in his torn white shirt and khaki pants, took the whole thing in his stride, handed us out little bundles of stamps, airmail stickers and, in due coure, little piles of change.
"It just shows you," said Dizzy, "that appearances can be just as deceptive with Africans as with English. He looks a complete moron, but he must actually be a sort of mathematical genius to cope with that lot."
From NOTHING HAPPENED AFTER ALL, Chapter 7, Hatching Plots.