Don's son was born after 1/6+1/7+1/12 of his life +5 years. The son died 4 years before don and lived half as long.
Son was born in the year (1/6+1/12+1/7)x+5
the son died in the year x-4
Subtracting the year of birth from the year of death- it's son's lifespan and half of fathers lifespan.
Solve this and you will get the answer