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Seven math problems: Happy Holidays, Winter Holidays, General Mobilization, Shanghai Science Edition, Seventh Grade Mathematics, North Women and Children Publishing House, P43.
2a+4b=4000, 3a+3b=4200, so a=800, b=600,

800x+600y=7400, x, y must be a positive integer, x∈,

(2) There are X middle school students and Y primary school students.

X = 1,Y = 1 1,X = 2,Y = 29/3,X = 3,Y = 9,X = 4,Y = 7,X = 5,Y = 17/3。

So * * * has x= 1, y =11; x=3,y = 9; x=4,y = 7; X = 7, y = 3, and * * * are four schemes.