An electrical circuit switches instantaneously from a 3 volt battery to a 18 volt battery 4 seconds after being turned on. Sketch on a sheet of paper a graph the battery voltage against time. Then fill in the formulas below for the function represented by your graph.

For $t <$ , $V(t) =$
For $t \ge$ , $V(t) =$ .

At what point or points is your function discontinuous?
$t =$
(Give your answer as a comma-separated list, or enter the word none if there are no discontinuities.)