Hydrology and Runoff

Between two adjacent isohyets of $100\text{ mm}$ and $80\text{ mm}$, the enclosed area is $A_1 = 40\text{ km}^2$. Average rainfall in this zone is $\frac{100 + 80}{2} = 90\text{ mm}$. Between $80\text{ mm}$ and $60\text{ mm}$ isohyets, the area is $A_2 = 60\text{ km}^2$ with average $70\text{ mm}$. Mean precipitation over the total $100\text{ km}^2$ area in mm is: