goi so cay ma moi lop 7A,7B,7C lan luot la a,b,c (a,b,c thuoc N*)
=>a/45=b/54=c/51
ma lop 7A va lop 7C trong nhieu hon lop 7B la 14 cay
=> a+c-b=14
ap dung ttinh chat day ti so bang nhau ta co
a/45=b/54=c/51=a+c-b/45+51-54=14/42=1/3
=>a/45=1/3=>a/45=15/45=>a=15
    b/54=1/3=>b/54=18/54=>b=18
    c/51=1/3=>c/51=17/51=>c=17
vay lop 7A trong duoc 15 cay
       lop 7B trong duoc 18 cay
      lop 7Ctrong duoc 17 cay