Imagine a family with:
How many people are in the family?
This is because
there is one grandmother/mother to a daughter who is a mom and a daughter who has a daughter who is the granddaughter.
Alice - Beth's Mother, Cat's Grandmother
Beth - Alice's Daughter, Cat's Mother
Cat - Beth's Daughter, Alice's Granddaughter
A grandmother is an old lady, her daughter a middle-aged wonen and small girl.
The old lady is the mother of the middle aged women. The middle aged women is the daughter of the small girl. So we have two mothers and two daughters.
The old lady is the grandmother of the small girl. Thus the girl is the granddaughter of the old women.