9 children, because each one has only one sister...
Eigth sons and one daughter=9 children.
8 son and 1 daughter = 9 children.
NINE. each son has one sister. that means the friend has One Daughter and Eight sons.
He has in all 9 children
9 children--> 8 sons + 1 daughter
Total 9 children
My friend has 9 children including 8 sons and 1 daughter