A formula is a machine with labelled sockets. $v = d/t$ takes a distance and a time; $a = \Delta v/t$ takes a change in speed and a time. The fastest way to pick the right machine is to ask what the story GIVES you — not what it asks for.

Steady motion, no speeding up? That is $v = d/t$ territory. Something gaining or losing speed? That is $\Delta v$, and $\Delta v$ means acceleration. Nothing has a mass in this chapter, so if you ever feel tempted by $F = ma$ here, the problem is telling you to reread it.