Typical rule with metric and English measurements. Following are two things commonly seen with college students when using a ruler to measure features on topographic maps.
First common error is that they'll measure something that's 3 cm in length but they'll report that it's 3 mm in length. Why? The ruler has mm printed on the metric scale so they think each number is a mm. By the way, I've had students tell me, in all seriousness, that they were never exposed to the metric system in public school. I have no idea if that's true or the student was just sleeping on those days but they swear it's all new and magical to them.
Second common error is that they'll measure something as 3 in plus, let's say, 3 tick marks and report it as 3.3 in. They don't realize that English units on virtually all rulers are divided into eighths (or sixteenths), not tenths, and they're not the same thing (3 3/8 = 3.375). Good thing they're going to college and not trying to be something useful like carpenters ("I just can't understand why these floor joists are all too short!").
Don't kids learn to read rulers in, like, third grade? Don't even get me started on protractors - half the class can't use those high-tech instruments (they measure the angle from the bottom of the protractor, not the 0° line, or measure from the right-side and call an acute 50° angle an obtuse 130°).
I'm not big on tattoos but here's one that might actually be useful...
From Descending Ashtray blog
Of course if we ever switch to metric he'll be S.O.L.