Family earnings :
?   12.50	: tax on wages
    2		: family persons

    20000.00	: John's gross earnings
%   tax on wages
= : John's tax on wages

    John's gross earnings
-   John's tax on wages
= : John's net earnings

    15000.0	: Jane's gross earnings
%   tax on wages
= : Jane's tax on wages

    Jane's gross earnings
-   Jane's tax on wages
= : Jane's net earnings

    John's net earnings
    Jane's net earnings
= : family earnings

Family expenses :
    John's cigarettes	: 5020
    John's gas		: 7160.6
    John's rent		: 10050
    John's food for dog : 525.30
= : John's expenses

    Jane's new car	: 15e+4
    Jane's gas		: 3500.55
    Jane's rent		: 7325.70
= : Jane's expenses

    John's expenses
    Jane's expenses
= : family expenses

Family balance :
    family earnings
-   family expenses
? : family balance
/   family persons
= : family balance per person