8

Set 12: Exponential Functions (Intermediate)

Explanation

Answer: A

A phone's battery percentage follows B(t)=100(0.92)tB(t) = 100(0.92)^t where tt is hours. After how many hours is the battery at 50%?

A.

Between 8 and 9 hours

✓ Correct
B.

Exactly 50 hours

C.

Between 2 and 3 hours

D.

After 10 hours

Detailed Explanation

Choice A is correct. Choice A is the correct answer. Solve 100(0.92)t=50100(0.92)^t = 50. 1. Simplify: (0.92)t=0.5(0.92)^t = 0.5. 2. Test Values: - t=8t=8: (0.92)80.51(0.92)^8 \approx 0.51 (above 0.5) - t=9t=9: (0.92)90.47(0.92)^9 \approx 0.47 (below 0.5) 3. Conclusion: Answer is between 8 and 9 hours. Strategic Tip: For SAT, estimation and testing values is often faster than logarithms. Choice B is incorrect because 50 is the target percentage, not the time. Choice C is incorrect because the decay is too slow for this short time. Choice D is incorrect because by hour 10, battery would be below 50%.

Key Steps:

The correct answer is Between 8 and 9 hours

Why others are wrong:
B: Choice B is incorrect and may result from a calculation error.
C: Choice C is incorrect and may result from a calculation error.
D: Choice D is incorrect and may result from a calculation error.

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