(a) Let Donald be denoted by H (Husband)
His wife be denoted by W (Wife)
His daughter be denoted by D (Daughter)
His son be denoted by S (Son)
The average age of 4 persons
==23
⇒H + W + D + S = 92
Again H = W + 4
So at the time of birth of his son, total age of his family = 64 years(32 + 28 + 4 + 0 = 64)
and presently the total age of his family = 92 years
It means total increase in age of the whole family = 28 years
Thus the average increase in age =
=7 years
It means the age of Donald = 39 years
and age of his daughter = 11 years
Therefore, the average age of Donald and his daughter is 25 years.