Answer: After 5 minutes, the plane is at 20,000 feet. It descended a total of 10,000 feet.
Step-by-step explanation:
f = feet above ground
r = rate of descent
m = minutes
f = 30000 - r · m
f = 30000 - 20000m
After five minutes:
f = 30000 - 2000(5)
f = 30000 - 10000
f = 20000