a, goi so can tim la a
ta co : a : 2,3,4,5,6 deu du 1
suy ra : a-1 : het cho 2,3,4,5,6
( a - 1) la boi chung cua 2,3,4,5,6
a-1 = { 60,120,180,240,300,360,420,480...}
mat khac ta co a chia het cho 7 va phai la so nho nhat
neu a-1=300 thi a=301 la so nho nhat thoa man yeu cau cua de bai
b, a= 2q + 1 = 3r + 1 = 4p +1 = 6s +1 = 7y