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.