We talked about residence time, giving a simple equation to find it: **F = M/T**, where F is the flow in and out, M is the stock, and T is the residence time. When using an equation like this, you need to pay attention to **the units that go with the values** you plug in.

Say the **flow in is 5 gallons per minute** (gal/min), and this is also equal to the flow out. **The stock, or size of the tank, is 500 gallons**. We easily plug in the numbers and find that the residence time T is 100.

But 100 what? **The residence time is 100 ***minutes*, and we know that by **doing the same math we did on the numbers on the words that go with them**.