Annals of Marginal Numeracy

Lifehacker is running the following “tip” to convert a time specified in 24-hour notation to one in 12-hour notation:

Subtract 10 then 2 to get the 12 hour clock time.

This is not so much a “tip” as a restatement of the definition. 24-hour times between 13:00 and 23:59 correspond to 12-hour times that are … yes, 12 less in the hours place. When challenged on the triviality of this algorithm, the author, Gina Trapani, responded:

I think because 10 and 2 are easier to subtract than 12 or 6.

The comments thread has been filling with well-deserved ridicule, which, to her credit, Trapani has been taking with good humor. Still, the only difference I can see between this “handy” technique and the good old-fashioned method called “subtracting 12” is that the old-fashioned method first subtracts 2 and then subtracts 10, instead of vice-versa. Apparently, the target Lifehacker audience consists of people who can hold in their memory numbers up through and including 14 but not numbers as exorbitantly large as 22.

Maybe I’m the target audience? As you may know, I have some trouble with mental arithmetic, and needing to use the 24h clock for some things is why I like my watch to have the extra numbers on it in small print: I find that actually doing the conversion quickly is difficult. To me, subtracting ten or two is easier than subtracting 12.

So I’m not sure the ridicule is well-deserved.

Perhaps the Lifehacker post might have been better had it described “subtract 10 and then subtract 2” as a means of implementing “subtract 12” rather than suggesting that this was a wholly alternative technique.

Maybe I’m just sensitive about these things… but I think “innumerate” and “bad at doing arithmetic in one’s head” shouldn’t be confused. It’s analogous to saying that people who can’t spell are illiterate. When I need to do a calculation, I know to use a calculator; bad spellers usually know to consult a dictionary.

Point well taken. I’m conflating two things: (1) actual functional innumeracy, in which people run screaming from numbers and cannot interperet or work with them at all, and (2) milder discomfort with mental arithmetic, a relatively weak horse sense for things mathematical. The existence of each is an indictment of our educational system. Only the former, however, should be considered pejorative.

The tip is a little bit silly, but the basic principle is sound: doing addition and subtraction in your head is substantially easier if you apply the operator by decreasing orders of ten. The normal method of starting from the ones has the twin disadvantages of creating the answer backward and requiring more involved “carrying over” or “borrowing.”