Respuesta :

1/3*1/4*1/7= 1/84
It is 9c3
There are 3!  = 6 ways he can pick the 3 oldest people
Total number of permutations  of any 3 from 9 = 9P3 =  9*8*7 = 504

so required probability = 6 / 504 = 1/84