I want to no how many tones of topsoil there is in an area 475square meters by 250milimeters in depth

Your title and your question do not match, which one do you want to know? IOf you want to KNOW how many TONS there are in acertain amount of soil it will depend on the type of soil and the moisture content. Answers.com - How much does dirt weigh (http://wiki.answers.com/Q/How_much_does_dirt_weigh)

250mm is 1/4 of 1000mm

so 475 x 1/4 = 120 cubic metres or thereabouts

And 1.5 tonnes of soil / sqm ( was the old way we worked stuff out )

So 180 tonnes of soil to be removed

That should read: 1.5 tons of soil per cubic metre