On John's 31st birthday his mother's age is 49. John adds the digits of his mother's age to get 4+9 = 13. He then repeats this process to get 1+3 = 4. Once he gets to a single digit he stops. He applies exactly the same process to his own age and gets 4 again.
He repeats this calculation for his next birthday, the birthday after that and so on. To his amazement, repeatedly adding the digits of his mother's age always gives the same value as repeatedly adding the digits of his own age. It seems that he and his mother share a special numerical relationship.
John then investigates whether he shares this numerical relationship with his father. On John's 31st birthday his father's age is 56. Adding the digits gives 5+6 = 11 and adding again gives 1+1= 2. Alas not! Furthermore, he discovers that he never does.
This age relationship between parent and child is not uncommon. What property must the parent's age have on the day the child is born if they are to share this numerical relationship?