Answer
$d=50t$
Work Step by Step
The car and truck are moving along perpendicular lines.
At any time t, their positions are vertices of a right triangle,
and the distance between them is the length of the hypotenuse.
Apply the Pythagorean theorem.
$d^{2}=(30t)^{2}+(40t)^{2}$
$d^{2}=900t^{2}+1600t^{2}$
$d^{2}=2500t^{2}$
$d=50t$