Given:
Christms bonus = 7800
total earn in year =89100
So only salary is:
[tex]\begin{gathered} salary=89100-7800 \\ =81300 \end{gathered}[/tex]In a year ( 12 months) salary is 81300 so per month salary is:
[tex]\begin{gathered} 12\text{ months salary is: 81300} \\ 1\text{ month salary is=}\frac{81300}{12} \\ =6775 \end{gathered}[/tex]So monthly salary is 6775.