Let's analyze each of the statements regarding changes to the annual membership fee.
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Statement A: "The annual membership fee at the fitness club decreased by 5%. An equivalent expression that represents this situation is m − 0.05m."
- A decrease of 5% means you pay 95% of the original fee, which can be expressed as: \[ m - 0.05m = 0.95m \]
- This statement is true.
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Statement B: "The annual membership fee at the fitness club decreased by 105%. An equivalent expression that represents this situation is m − 1.05m."
- A decrease of 105% would imply paying back more than what you originally paid, which is nonsensical in this context. The calculation is: \[ m - 1.05m = -0.05m \]
- This statement is false.
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Statement C: "The annual membership fee at the fitness club increased by 5%. An equivalent expression that represents this situation is m + 0.05m."
- An increase of 5% means you pay 105% of the original fee, expressed as: \[ m + 0.05m = 1.05m \]
- This statement is true.
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Statement D: "The annual membership fee at the fitness club increased by 105%. An equivalent expression that represents this situation is m + 1.05m."
- An increase of 105% would mean paying 205% of the original fee, expressed as: \[ m + 1.05m = 2.05m \]
- This statement is false.
Therefore, the true statements are A and C.