3. Option 1.

In the Ivanov family, both the mother and the father, and their three children, were born on April 1st. When the first child was born, the parents' combined age was 45 years. The third child in the family was born a year ago, when the sum of the ages of all family members was 70 years. How old is the middle child now, if the sum of the ages of the children is 14 years?
Answer: 5.

Solution.

If the first child is older than the second child by $x$ years, and the middle child is older than the third child by $y$ years, then $70-45=3(x+y)+y$, because the age of each parent and the eldest child increased by $(x+y)$ years by the time the third child was born, and the age of the second child increased by $y$ years. Similarly, $(x+y+1)+(y+1)+1=14$. We have the system of equations $3x+4y=25, x+2y=11$, from which we find $x=3, y=4$.