Answer:
10 days
Step-by-step explanation:
On day 1, Brenda reaches 3000 ft.
On day 2, Brenda reaches 4000 ft, having had to start from 1000 ft.
On day 3, Brenda reaches 5000 ft, having had to start from 2000 ft.
...
On day 10 Brenda reached 12000 ft, having had to start from 9,000 ft.
Brenda reaches a height that is 1000·(day+2).
It takes Brenda 10 days to reach the 12,000 ft summit.
