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\noindent {\bf Problem statement} A small object of unknown
temperature was placed in a large room that had the fixed temperature
$30^{\circ}\, {\rm C}$. After 10 minutes, the object's temperature is
$-10^{\circ}\, {\rm C}$, and after an additional 10 minutes, the
object's temperature was $-5^{\circ}\, {\rm C}$. What was the initial
temperature of the object? (Assume that the temperature obeys Newton's
law of cooling: the rate of change of the temperature of the object is
proportional to the difference in temperature between the object and
the constant room temperature.)

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