Age 16 to 18 Challenge Level:
Students: Don't be deterred if you have not learnt the method of
solving differential equations by separation of variables because
all you have to do is to rearrange the equation in the form $$\int
f(v) dv = \int f(x) dx$$ so that you can integrate both sides and
the rest is easy integration.
Teachers: This is an example you might use to launch into work on
learning the method of separation of variables.