1. The problem statement, all variables and given/known data Apond initially contains 1,000,000 gal of water and an unknown amount of an undesirable chemical. Water containing 0.01 g of this chemical per gallon flows into the pond at a rate of 300 gal/h. The mixture flows out at the same rate, so the amount of water in the pond remains constant. Assume that the chemical is uniformly distributed throughout the pond. (a) Write a differential equation for the amount of chemical in the pond at any time. 2. Relevant equations This should be done without a prerequisite of any formula 3. The attempt at a solution Well, I do have the solution which says let q(t) be the amount chemical at any time. Hence, the concentration of this chemical in the water at any time is q(t) / 1000000. (This part is understood). 300*(0.01) is the amount of chemical coming into the pond every hour. (This part is fine too).. Now, the last and most important part ... 300 q(t) / 100000 is the rate at which the chemical leaves the pond per hour. What?? How did they give this claim?? I mean, in the text it is written that " The mixture flows out at the same rate, so the amount of water in the pond remains constant. ". I am completely perplexed here. Honestly, i need to know some basics of DE. If anyone would give me a hand on how to approach the answer, I would owe him/her big time. Thanks a lot for your time!