To find out how many seconds it would take for him to reach the ground, one would simply have to divide the distance by the rate at which the skydiver is travelling per second.
We know that the skydiver is travelling 1200 feet for ever 5 seconds. To find how many feet he is travelling per second, we simply need to divide the distance by the time.
1200 / 5 = 240
He is falling 240 feet per second.
Now, to find out how many seconds it will take him to reach the ground, we just need to divide that rate into the distance.
10000 / 240 = 41 2/3
It will take the skydiver 41 2/3 seconds to reach the ground.
Hope this helped =)