Answer:
I assume you want to know the rate of descent per minute
Rate of descent = -1000 ft/min
Where the negative sign signifies that the altitude is decreasing.
The rate of descent is 1000 feet per minute
Explanation:
At the beginning, the plane is at an altitude of 29,000 ft
After 20 minutes, it is at 9000 ft,
So, the rate is,
Rate of descent = (9000 - 29000)/20
Rate of descent = -20000/20
Rate of descent = -1000 ft/min