This particular problem is attributed to Alcuin. The wording does not give sufficient information to answer the question without making legal assumptions. Please share your assumptions with your solution.
A dying man left 960 shillings and a pregnant wife. He directed that if a boy was born, he should receive threequarters of the whole and the child's mother should receive onequarter. But if a daughter was born, she would receive seventwelfths, and her mother fivetwelfths. It happened however that twins were born  a boy and a girl. How much should the mother receive, how much the son, and how much the daughter?
Since equality is equity, apply the pari passu rule first, and divide the estate in half before computing the shares severally:
Mother gets 960/2*1/4+960/2*5/12 = 320
Boy gets 960/2*3/4= 360.
Girl gets 960/2*7/12 = 280.
The link given shows a much more complicated way of reaching the same result.

Posted by broll
on 20170725 22:27:43 