百分比增加还有减少怎样计算?请告诉我这题如何计算there are 40 students in S1A.if 2 new students join the class,what is the percentage increase in the number of S1A students?

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百分比增加还有减少怎样计算?请告诉我这题如何计算
there are 40 students in S1A.if 2 new students join the class,what is the percentage increase in the number of S1A students?