Henry Ernest Dudeney/Puzzles and Curious Problems/48 - Robinson's Age

by : $48$

 * Robinson's Age
 * Robinson said.
 * My brother is two years older than I,
 * my sister is four years older than he,
 * my mother was $20$ when I was born,
 * and I was told yesterday that the average age of the four of is is $39$ years.


 * What was Robinson's age?