Answer
$v=25.7ms^{-1}$
Work Step by Step
Let $v$ be the velocity when she reaches the light.
Distance travelled till she steps on brakes
$s = vt= 20*1 = 20m$
remaining distance is $200m - 20m =180m$ and remaining time $15s-1s=14s$
let $a$ be her deceleration
$a= (v-20)/14$
we have
$2as =v^2 -u^2$
$ 2*(v-20)14 *180= v^2 - 20^2 =(v-20)(v+20)$
$180/7 = v+20$
$v=25.7ms^{-1}$
