Answer
$25t$
where $t\geq 0$.
Work Step by Step
We know that the distance covered is the product of the time passed and the average speed. Furthermore, their routes have a right angle between them, thus according to Pythagorean's Theorem we, get:
$D^2=(15t)^2+(20t)^2$
$D=\sqrt{(15t)^2+(20t)^2}=\sqrt{625t^2}=25t$
where $t\geq 0$.
