33. In a certain factory, 27 masters collectively mentor 40 apprentices. Each master can mentor one, two, or three apprentices. If the number of masters mentoring one apprentice is twice the number of other masters, then the number of masters mentoring two apprentices is $\qquad$.

将上面的文本翻译成英文，请保留源文本的换行和格式，直接输出翻译结果。
answer: 5