Answer:
15 seconds.
Step-by-step explanation:
∵ The distance covered by plane in first second = 100 ft,
Also, in each succeeding second it climbs 100 feet more than it climbed during the previous second,
So, distance covered in second second = 200,
In third second = 300,
In fourth second = 400,
............, so on....
Thus, the total distance covered by plane in n seconds = 100 + 200 + 300 +400......... upto n seconds
( Sum of AP )
Suppose the distance covered in n seconds is 12,000 feet,
∵ n can not be negative,
Hence, after 15 seconds the plane will reach an altitude of 12,000 feet above its takeoff height.