The Father

Mary is 21 years older than her son. After 6 years, she will be 5 times older than him. Where is the father? 

Let M be the age of the mother and S be the age of the son. We have M = S + 21 and M + 6 = 5(S + 6). We solve the system and get S= -3/4. Therefore right now the son just got conceived and the father is with the mother.