It takes 12 days for the water level to change -16 feet
Solution:
Given that
During a drought, the water level of a lake changes
feet per day.
Let assume number of days required for change of water level to -16ft = "x"

So change of water level in "x" days is:

As we assumed that change in “x” days in water level = -16feet

Hence it takes 12 days for the water level to change -16 feet