Honors Algebra II: Exponential Growth & Decay Problems

Determine whether each problem is exponential growth or decay. Then write the formula for the problem.
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Find a bank account balance if the account starts with $100, has an annual rate of 4%, and the money is left in the account for 12 years.
Exponential growth, y = 100(1 + 0.04)¹²
In 1985, there were 285 cell phone subscribers in a small town. The number of subscribers increased by 75% per year after 1985. How many cell phone subscribers were there in 1994?
Exponential growth, y = 285(1 + 0.75)⁹
Each year the local country club sponsors a tennis tournament. Play starts with 128 participants. During each round, half of the players are eliminated. How many players remain after 5 rounds?
Exponential decay, y = 128(1 - 0.5)⁵
During normal breathing, about 12% of the air in the lungs is replaced after one breath. Write a model for the amount of original air left in the lungs if the initial amount of air in the lungs is 500 mL. How much of the original air is present after 240 breaths?
Exponential decay, y = 500(1 - 0.12)²⁴⁰
You have inherited land that was purchased for $30,000 in 1960. The value of the land increased by approximately 5% per year. What is the approximate value of the land in 2011?
Exponential growth, y = 30,000(1 + 0.05)⁵¹
You drink a beverage with 120 mg of caffeine. Each hour, the caffeine in your system decreases by about 13%. How much caffeine will you have left in your system after 4 hours?
Exponential decay, y = 120(1 - 0.13)⁴