Draw a line that's a third of a meter.

Do it with a fifth of a yard ;-)

Like, 100/3 = 33 cm? That's supposed to be difficult?

Or do you mean that it's difficult to know how to draw a third of a meter?

Why, is drawing a line that's one meter, or one yard, or one foot any easier?

Without a measure I can't draw accurately any of them.

I guess they were referring to how a third of a yard would be a foot or 12 inches and that most people have an intuitive understanding of such common lengths. Also, because of the factors involved, “simple” fractions (e.g. ½, 1/3, ¼, ...) often end up at integral lengths.

But intuitive understanding of certain lengths is certainly not limited to the Imperial system. I frequently use A4 paper for measuring if I don't have a ruler at hand. I know the span between the tip of my thumb and pinky is pretty much 21 cm and so on. 8.2 inches probably wouldn't be much handier either.