Answer
$t = \frac{0.20~D}{c}$
Work Step by Step
We can find the total distance $d$ as measured from the rest frame of the starship:
$d = \frac{4D}{\gamma}$
$d = \frac{6D}{30}$
$d = 0.20~D$
We can find $\beta$:
$\gamma = \frac{1}{\sqrt{1-\beta^2}}$
$\sqrt{1-\beta^2} = \frac{1}{\gamma}$
$1-\beta^2 = \frac{1}{\gamma^2}$
$\beta^2 = 1-\frac{1}{\gamma^2}$
$\beta = \sqrt{1-\frac{1}{\gamma^2}}$
$\beta = \sqrt{1-\frac{1}{30^2}}$
$\beta = 0.999444$
We can find the travel time as measured from the rest frame of the starship:
$t = \frac{0.20 D}{0.999444~c}$
$t = \frac{0.20~D}{c}$
