Problem 4.5. Mom and Dad have two children: Kolya and Tanya. Dad is 4 years older than Mom. Kolya is also 4 years older than Tanya and half as old as Dad. How old is each of them if the total age of all family members is 130 years?
Answer: Tanya is 19 years old, Kolya is 23 years old, mom is 42 years old, and dad is 46 years old.

Solution. Let's mentally increase the age of mom and Tanya by 4 years. Then the age of two family members will be equal to dad's age, and another two will have half of dad's age. Then the total age of all family members is $130+4+4=138$ years, and it should be equal to three times dad's age. Therefore, dad should be $138: 3=46$ years old. From this, it's easy to understand that mom is $46-4=42$ years old, Kolya is $46: 2=23$ years old, and Tanya is $23-4=19$ years old.