George and Marge married after their respective divorces. Both had children from their previous marriages. Currently 9 children live with them. George is the father of 4 of them and Marge is the mother of 6.
How many children have had fruit from their current marriage?
If they had not had children in common, they would currently live with 4 + 6 = 10 children. Since they live with 9 children, they must have had 10 - 9 = 1 in common.