$($a$)$ A tank contains $7000$ L of pure water. Brine that contains $20$ g of sait per liter of water is pumped into the tank at a rate of $25\frac{L}{m}in.$ Show that the concentration of salt t minutes tater $($in grams per liter$)$ is

$C(t)=\frac{20t}{280t}$

After $t$ minutes $25t$ liters of brine with $20g$ of salt per liter has been pumped into the tank, so it contains

$$ liters of water and $$ grams of salt. Therefore, the salt concentration at time $t$ will be the following,

$C(t)=\frac{500t}{700025t}=\frac{20t}{280t}\frac{2}{L}$

$($b$)$ What happens to the concentration as $t?$

$\underset{t}{lim}C(t)=,\frac{g}{L}$