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## Homework Statement

A lava beds cool very slowly, remaining warm for years. Calculate the rate of heat conduction per square metre through a 30 m thickness of rhyolite, if the surface is 400 degrees celcius and its interior side is at 3100 degrees celcius. The thermal conductivity of rhyolite =0.84 Wm^-1 degrees celcius^-1.

Answer must be 75.6 W/m^2

## Homework Equations

[tex]P=kA \left| \frac{dT}{dx}\right|[/tex]

(The rate is P, k is thermal conductivity, dT/dx is temprature gradient)

## The Attempt at a Solution

dT/dx = 3100-400 = 2700

[tex]0.84 \times 30 \times(2700) = 68040[/tex]

Can anyone show me the problem? ...because the right answer is 75.6!