Saving an old lady

You are trying to pull an old lady out of the way of an oncoming truck. You are able to give her an acceleration of 20 m/s². Starting from rest, how much time is required in order to move her 2 m?

First we organize our thoughts:

Variables given: Dx, a, v_o

Variables desired: Dt

Irrelevant variables: v_f

Consulting the triangular chart above, the equation we need is clearly Dx = v_oDt + 1/2 aDt², since it has the four variables of interest and omits the irrelevant one. Eliminating the v_o term and solving for Dt gives