Five years ago, the average age of a family of four was 26 years. Since then a baby has been born, and today the average age of the family of five is 25 years. How old is the baby today?

Explanation

1. 01

   Five years ago the four members totalled $4 \times 26 = 104$ years.

2. 02

   Each has aged 5 years, so today they total $104 + 20 = 124$ years.

3. 03

   Today's family total = $5 \times 25 = 125$ years, so the baby is $125 - 124 = 1$ year old.