The age of Mr and Mrs Snow's children have an interval of two years each. So, if the first child is 'x' years of age, the second child will be (x + 2) years of age, third child will be (x + 4) years of age and fourth child will be (x + 6) years of age.
The sum of the ages of the children is 36
So,
x + (x + 2) + (x + 4) + (x + 6) = 36
4x + 12 = 36
4x = 36 - 12
4x = 24
x = 24 ÷ 4
x = 6
The first child is x years old = 6 years old
The second child is (x + 2) years old = 6 + 2 = 8 years old
The third child is (x + 4) years old = 6 + 4 = 10 years old
The fourth child is (x + 6) years old = 6 + 6 = 12 years old
The sum of their ages = 6 + 8 + 10 + 12 = 36