Let the number of children in each family be x. Thus the total number of children in both the families are 2x Now, it is given that 3 tickets are distributed amongst the children of these two families. Thus, the probability that all the three tickets go to the children in family B =
xC3
2xC3
=
1
12
⇒
x(x−1)(x−2)
2x(2x−1)(2x−2)
=
1
12
⇒
(x−2)
(2x−1)
=
1
6
⇒x=5 Thus, the number of children in each family is 5.