1. Define the variables:
Let the age of the youngest child be x years.
Since the children are born at intervals of four years, the ages of the other children will be x + 4, x + 8, x + 12, and x + 16.
2. Set up the equation:
3. Solve the equation:
5x + 40 = 80
5x = 80 - 40
5x = 40
x = 40 / 5
x = 8
4. Calculate the age of the eldest child:
Therefore, the age of the eldest child is 24 years.