even fair distribution

[Guest]

Hi Mathematicians,

I have a question how to evenly distribute amount of days between divorced parents so that each parent gets the same amount of overnights. Please propose a solution based on the following:
Spring break is 10 days. Each parent is to get 5 days.
however the Father requests to stay with child full consecutive 9 overnights,
The mother is willing to accept that under condition that she could use those 9 consecutive overnights with the child as well in the future.
The Mother owes the Father 2 overnights with a child that she is willing to give up so that Father can use them towards his 9 overnights.
Out of 9 overnights:
5 automatically belong to Father
2 he gets from Mom for the days she owed him from past with the child
2 he requests to get additionally from Mom so that he gets total of 9 overnight.

How many overnights should the Mother get for giving up spring break plans to be also fair to Mother and to keep an equal distribution for each parent. Please write an explanation from a mathematical point of view.

[mycalsuite]

Mother receives one night, but Father receives nine (5 fair share + 2 owing + 2 more). In the future, Mother should get four nights in a row to balance the total overnights in order to maintain fairness.