Answer
6.12 s
Work Step by Step
Let up be the positive direction.
The extra time taken by the pellet from gun A is the time taken for it to go up and come back down to the cliff edge. Its velocity when it comes back to the cliff edge is -30.0 m/s due to symmetry
$v=u+at$
$t=\frac{v-u}{a}$
$t=\frac{(-30.0)-(30.0)}{(-9.8)}\approx6.12\; s$
