Answer:
t = 10 s
Step-by-step explanation:
Given :
Altitude, h = 490 m
Initial velocity of a plane, u = 250 m/s
To find :
The time required for the packet to hit the ground. When it hits the ground its velocity will be 0. Let t is the required time. It can be calculated as follows :

So, it will take 10 s to reach the ground.