Question 807367
Concept is R*t=v;
R is fill or empty rate in vessel per time, whatever units are needed.
r is time, in whatever unit needed.
v is vessel or the tank as a unit itself, in whatever volume unit if necessary, but the focus may be on "vessel" as a unit.


Your example's rate for filling is F.  The rate for emptying is E for Empty or Evacuate.  {{{F>0}}} and {{{E<0}}}.  The actual rate occurring will be for emptying the tank.  This rate is F+E tanks per time.  This rate will be negative because E<F and |E|>|F|.


Your example starts with v=1, but you can call this a reference point of zero if you find it simpler.  


{{{highlight((F+E)t=-1)}}}
Solving for t for this emptying to be done gives {{{highlight(t=-(1/(F+E)))}}}.