Among the 40 students in Class 5 (1) of the school, the oldest is 13 years old and the youngest is 11 years old, so how many of them must be born in the same year and the same month.
学校五(一)班40名学生中，年龄最大的是13岁，最小的是11岁，那么其中必有多少名学生是同年同月出生的．
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