Answer
$8.7h$
Work Step by Step
The required number of hours can be determined as follows:
$t=\frac{Q}{P}$
$\implies t=\frac{cm\Delta T}{P}$
$\implies t=\frac{c\rho Ah\Delta T}{P}$
$\implies t=c\rho h\Delta T\frac{1}{P/A}$
We plug in the known values to obtain:
$t=4190(1000)(0.3)(25-15)\frac{1}{400}$
This simplifies to:
$t=31425s=8.7h$
