Know how many beans make five
A person who knows how many beans make five is British colloquial speak for a person who has their wits about them or who knows their stuff, and dates in this format from the late 18th/early 19th century. Sometimes the expression is 'knows how many blue beans make five', although giving the beans a colour is increasingly rare these days. It derives from an earlier expression 'to know one's beans' which means to know one's topic or subject, whatever it may be, and which dates from the late 1600s. (The beans are thought to refer to the beans or beads on an abacus. If you did not know how to use an abacus, then you did not know your beans.) An even earlier expression, 'know how many numbers are five' is found in Thomas Shelton's translation of Cervantes' Don Quixote in 1612, where an innkeeper accuses Don Quixote of talking to him 'as if I knew not how many numbers are five'. The question 'How many beans make five?' has been a been a childhood quiz in Britain since the early 20th century, to which the answers vary from 'one bean, two beans, a bean and a half and half a bean' to 'bean, bean, bean and a half, half a bean and a bean' or 'a bean, another bean. a bean and a half, half a bean, plus a bean' etc. As long as all the answers add up to five, the children pass the test.