goi so hoc sinh la a
so hoc sinh cua khoi 6 khi xep 2 3 4 5 deu thua 1 nguoi => a-1 chia het cho 2 3 4 5
2 = 2 . 1
3 = 3 . 1
4 = 2^2
5 = 1 . 5
BCNN ( 2 3 4 5 ) = 2^2 . 1 . 3 . 5 = 60
=> BC ( 2 3 4 5 ) = B ( 60 ) = 0 60 120 180 240 300 .......
a-1 = 120
a = 119
vay so hs khoi 6 la 119