The vertices of an equilateral triangle divide the circumference into 3 equal arcs. Therefore, the arc ABC is 2/3 of the circumference. 24/(2/3) = 36.

This is kind of tricky, because we're used to seeing circumference expressed in terms of pi. If I were going too quickly, and 36 were an option, I wouldn't be surprised if I picked that and moved on. Luckily it's not an answer, so it's easy to realize that you need to divide 36 by pi. pi~3, 36/3 = 12. Since pi is slightly larger than 3, we know the answer must be slightly smaller than 12, so the answer is 11.