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A metallic sphere cools from 50°C to 40°C in 300 s. If atmospheric temperature around is 20°C, then sphere’s temperature after the next 5 minutes will be close to:

1. 33°C
2. 31°C
3. 35°C
4. 28°C

Correct answer: 33°C

Solution

Newton cooling (average-temp form): (50-40)/300 = k(45-20) -> k=1/750. Next: (40-T)/300 = k((40+T)/2 - 20) = (1/750)(T/2). Solving: 200 = 6T -> T = 33.3C ~ 33C.

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