A tank contained 80 liters of water. The function represents the relationship between , time in minutes, and the amount of water in the tank in liters. The equation defines this function.

1. How much water will be in the tank after 13 minutes?
2. How many minutes will it take until the tank has 5 liters of water?
3. In this situation, what information can we gain from the inverse of function ?
4. Find the inverse of function . Be prepared to explain or show your reasoning.
5. How would the graph of the inverse function of compare to the graph of ? Describe or sketch your prediction.

In 2004, less than 5% of the homes in the United States relied on a cell phone instead of a landline phone. Since then, the percentage of homes that used only cell phones has increased.

Here are the percentages of homes with only cell phones from 2004 to 2009.

years since 2004	percentages
0	4.4
1	6.7
2	9.6
3	13.6
4	17.5
5	22.7

Graph of data, origin O. Horizontal axis, 0 to 5, years since 2004. Vertical axis, 0 to 24, percentage. Data points at 0 comma 4 point 4, 1 comma 6 point 7, 2 comma 9 point 6, 3 comma 13 point 6, 4 comma 17 point 5, and 5 comma 22 point 7.

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1. Suppose a linear function, , gives us the percentage of homes with only cell phones as a function of years since 2004, .

   Fit a line on the scatter plot to represent this function, and write an equation that could define the function. Use function notation.

2. Use your equation to find the value of . Then, explain what it means in this situation.
3. Use your equation to solve for . What does the solution represent?
4. Suppose we want to know when the percentage of homes with only cell phones would reach 50%, 75%, or 100% (assuming that the trend continues and the function stays valid). What equation could be written to help us find the years that correspond to those percentages? Show your reasoning.