brazil-enem 2013 Q172

brazil-enem · Other · enem__day2 Solving quadratics and applications Geometric or real-world application leading to a quadratic equation
The temperature $T$ of an oven (in degrees Celsius) is reduced by a system from the moment it is turned off ($t = 0$) and varies according to the expression $T(t) = -\frac{t^{2}}{4} + 400$, with $t$ in minutes. For safety reasons, the oven lock is only released for opening when the oven reaches a temperature of $39^{\circ}C$.
What is the minimum waiting time, in minutes, after turning off the oven, for the door to be opened?
(A) 19.0 (B) 19.8 (C) 20.0 (D) 38.0 (E) 39.0 
The temperature $T$ of an oven (in degrees Celsius) is reduced by a system from the moment it is turned off ($t = 0$) and varies according to the expression $T(t) = -\frac{t^{2}}{4} + 400$, with $t$ in minutes. For safety reasons, the oven lock is only released for opening when the oven reaches a temperature of $39^{\circ}C$.

What is the minimum waiting time, in minutes, after turning off the oven, for the door to be opened?

(A) 19.0
(B) 19.8
(C) 20.0
(D) 38.0
(E) 39.0
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