Let's start by defining the cost functions for both Plan A and Plan B in terms of the number of movies watched, \(x\).
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Plan A: \[ A(x) = 20 + 1x = 20 + x \]
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Plan B: \[ B(x) = 10 + 3x \]
Next, we need to find the interval for which Plan A is cheaper than Plan B. This can be expressed as an inequality: \[ A(x) < B(x) \]
Substituting the functions we defined earlier, we get: \[ 20 + x < 10 + 3x \]
Now, we'll solve for \(x\):
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Subtract \(x\) from both sides: \[ 20 < 10 + 2x \]
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Subtract 10 from both sides: \[ 10 < 2x \]
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Divide both sides by 2: \[ 5 < x \quad \text{or} \quad x > 5 \]
Thus, Plan A is cheaper than Plan B when the number of movies watched, \(x\), is greater than 5.
Graphing the Functions
To graph the functions \(A(x)\) and \(B(x)\):
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Plan A (\(A(x) = 20 + x\)):
- When \(x = 0\), \(A(0) = 20\)
- When \(x = 5\), \(A(5) = 25\)
- When \(x = 10\), \(A(10) = 30\)
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Plan B (\(B(x) = 10 + 3x\)):
- When \(x = 0\), \(B(0) = 10\)
- When \(x = 5\), \(B(5) = 25\)
- When \(x = 10\), \(B(10) = 40\)
Plotting the Points
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For Plan A (Linear with a slope of 1 and y-intercept 20):
- Points: (0, 20), (5, 25), (10, 30)
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For Plan B (Linear with a slope of 3 and y-intercept 10):
- Points: (0, 10), (5, 25), (10, 40)
Conclusion
The interval for the number of movies watched \(x\) such that Plan A is cheaper than Plan B is: \[ \text{For } x > 5 \]
In summary, Ella should choose Plan A when she watches more than 5 movies in a month.