Resposta :
É só igualar P = 400 e resolver a exponencial. Assim:
[tex]P(t)=25.2^t\\\\400=25.2^t\\\\2^t=\frac{400}{25}\\\\2^t=16\\\\2^t=2^4\\\\\boxed{t=4~~horas}[/tex]
[tex]P(t)=25.2^t\\\\400=25.2^t\\\\2^t=\frac{400}{25}\\\\2^t=16\\\\2^t=2^4\\\\\boxed{t=4~~horas}[/tex]