Goi so hoc sinh khoi 6 cua truong do la a
(a thuoc N*,hoc sinh)
Theo de bai ta co:  
a:12           } =>a thuoc BC(12,15,18)
a:15             va a lon hon 200 hoac be hon 400
a:18
a lon hon 200 hoac be hon 400
12=2^2.3
15=3.5
18=2.3^2
BCNN(12,15,18)=2^2.3^2.5=180
BC(12,15,18)=B(180)={0;180;360;540;720...}
Ma a lon hon 200 hoac be hon 400
=>a=360(tm)
Vay so hoc sinh khoi 6 cua truong do la 360 hoc sinh.