Need help to auto calculate annual leave/sick leave day

Very new in this board. If I post this question in the wrong forum please bear with me.
There is about 20 staffs in our department and I have made a spreadsheet with 12 tabs for each month of the year Jan-Dec. If each staff have 15 days annual leave each year and how can I make a formular that can automatically calculate the balance of annual leave and entered in the tabs of the following months. For example if staff A used 5 days in Jan how can the balance of 10days be entered in the tabs from Feb-Dec. If he use another 5 days in July the remain balance of 5 days be automatically entered in the tab from Aug to Dec. Likewise if staff A took 2 days sick leave in Jan, and 2 days in Mar how can this sick days be automatically sum up in the months after. I am doing this manually at the moment and it is very time consuming and also very easy to make mistake.
Thanks and sorry for this long questions

Andy,
simply create a table in each tab with the formula that will calculate the balance after each month and that will carry over to the next tab. See attached example for months Jan - Apr to give you an idea.

Alternatively, you can put everything on one sheet (easier, in my opinion) and simply calculate the balance. See "Alternative" tab. Hope this helps.