Example $1$: $10wt\%$ ethanol in water and $30wt\%$ ethanol in water are flowing into a mixer at flow rates of $100kg$ and $50kg$ per hour $(100k\frac{g}{h},50k\frac{g}{h}),$ respectively. After being mixed, what will be the concentration $($wt$\%)$ of the aqueous ethanol solution at the mixer outlet, assuming that all the fluid flows out?