If the pulse width were cut in half in the given circuit, the voltage across the resistor would be ______ at the end of the pulse.

ANS:C - 0.46 V

Since this is an RC circuit voltage in the resistor decreases with time as the voltage in the capacitor increases, therefore we have to use the formula for exponentially decreasing value V=Vin(e^(-t/TC));

t = 500μs/2 = 250μs.
TC = RC = (0.1μF)(680 Ω) = 68μs.
V = (18)(e^-(250μs/68μs)) = 0.4556187789V i.e 0.46V.