Originally Posted by

**scorpion007** Hi, can you explain how i might solve this problem?

A tank initially holds 100 litres of water. A sugar solution containing 0.2 kg per litre is being run into the tank at 3 litres per minute. The liquid in the tank is being continuously stirred and, at the same time, liquid from the tank is being pumped out at 2 litres per minute. If there are $\displaystyle m$ kg of sugar dissolved in the solution after $\displaystyle t$ minutes, then $\displaystyle \frac{dm}{dt}$ is equal to:

Apparently the answer is:

$\displaystyle \frac{3}{5} - \frac{2m}{100+t}$

But i dont know how they got it.