Affiliate Marketing Secrets

Homer Simpson wants to mass market his world famous Flmaing Moe cocktail made of tequila, schnapps, creme de menthe, plus the secret ingredient – Krusty’s Non Narkotik Kough Syrup for Kids. He starts by filling a tank with 80 liters of alcohol mix. Next, he pumps an alcohol solution containing 0.5 kg per liter of cough syrup into the tank at a steady rate of 4 liters per minute. The well-mixed solution leave the tank at the same rate.
A) Find the amount of cough syrp in the tank at any time t.
B) The drink tastes best when the cough syrup concentration is 0.2 kg/l. Find the time t when Homer can bottle a Flaming Moe Premium for his personal stash.
If anyone could explain how to arrive at the solution, I would greatly appreciate it!