EXMPLE $19$

Consider a tank containing $500$ gal of a coal$-$oil slurry in figure $11.$ this slurry has initially $200lb$ of fine coal particles. If pure oil enters the tank at the rate of $10ga\frac{l}{m}in,$ and the coal$-$oil slurry leaves the tank at the same rate so that the tank contains $500$gal of a slurry of changing concentration,

Determine the amount of coal left in the tank afer $20$min. Make any logical assumptions.