How is Mean Energy Density in the Serengeti Calculated?

[killbot2000]

Homework Statement

It has been estimated that a total weight of 4x10^7 kg of ungulates die in the Serengeti each year (Houston, 1979). We assume that these have an mass-specific energy content of 7x10^6J kg^-1 (Peters, 1983), and that the Serengeti stretches over 25,000 km^2 (Sinclair and Norton-Griffiths, 1979) This gives a mean energy density of 31 Jm^-2 day^-1

Homework Equations

The Attempt at a Solution

I'm struggling to see how the authors get their final figure of 31 joules per metre squared per day. Presumably it's the total weight multiplied by the mass specific energy content and that divided over the land area. I don't get their answer when I do this.


Any insight would be much appreciated.

It's from the appendix of this paper http://www.sciencedirect.com/science/article/pii/S0022519304000542

 

[mfb]

WolframAlpha gets the same result. Where is the problem? Unit conversions?

 

[killbot2000]

No idea where I was going wrong but thanks for that.