# Question: when the rate of inflow of a substance into a...

###### Question details

When the rate of inflow of a substance into a lake
(F_{in}), the atmosphere, the ocean, or any other “box” is
equal to the outflow (F_{out}) of that substance, then the
amount, or “stock” of that substance in the box will be constant.
This is called a “steady state” or “equilibrium”. The ratio of the
stock in the box to the flow rate (in or out) is called the mean
residence time (MRT). Thus the steady state condition is
F_{in}=F_{out}. Letting M be the stock (e.g., mass)
and MRT be the mean residence time, M/F_{in} =
M/F_{out} = MRT.

**Steady State Models Part 1.** A population of
fish in a lake is in a steady state. Each year 670 fish are born.
The average residence time for the fish in the lake is 7 years. How
large is the population of fish in the lake? Draw the steady state
box model

**Steady State Models Part 2.** Suppose there is a
stable population of 80 frogs in a wetland. Each year 15 get eaten
by birds, 4 die of old age, and 20 new frogs are born into the
wetland. What is the average residence time of a frog in the
marsh?. Draw the steady state box model