\Omega =c5 cua 40= 658008
nhom truong la nam co 22 cach
th1:1 nam.3nu co:22.18=396 cach
th2: 2 nam 2 nu :231.231=53361 cach
th3: 3 nam 1 nu: 1540.18=27720 cach
vay co n(a) = 22+396+53361+27720=81502 cach
=>p(a)=0.124