They also had to carry out quite complex mathematical operations several times a day, just to go about their everyday lives.
In his book On the abacus, Piero gives an exercise:
There are two men who want to barter; one of them has cloth and the other has wool. The piece of cloth is worth 15 ducats and he puts it up for barter at 20 and also wants one-third in money. And a cento of wool is worth 7 ducats in money. What price must [the man with wool] put it up for barter so that neither will be cheated?
This is the kind of thing you'd have to do in your head, on the spot, in public, while bargaining with an opponent.
You'd do it using the rule of three, which Piero gives as follows:
The rule of three says that the thing one wants to know must be multiplied by that which is not similar and the result it produces must be divided by the other; and the result is of the nature of that which is not similar, and the divisor is always similar to the thing one wants to know.
I think it's fair to say to that your average person in the 15th century West had a set of skills that your average person in the early 21st century West doesn't. As Michael Baxandall has pointed out, this has implications for how they looked at pictures, particularly early perspective pictures.