SOLUTION: Rita was 25 when her first son was born. Today the sum of their ages is 105. How old is Rita and her son?

Let the son's age today be S

>>...Rita was 25 when her first son was born...<<

This tells us that Rita is 25 years older than her son,
therefore,

Rita's age today is (S+25)

>>...Today the sum of their ages is 105...<<

Rita's age today +  Son's age today = 105

(S+25) + S = 105

Solve that and get S = 40.

So the son is 40 and Rita is 25 years older or 65.

Checking:

40 years ago the son was just born and Rita was 25.

Edwin