Bourne shell question

[The Ancient Programmer]

	How does one do a simple computation in a shell script?
The c-shell does it very neatly. 
Running:
	#!/bin/csh
	set a = 10
	set b = 1
	@ c = $a - $b
	echo "a=$a, b=$b, c=$c"

produces: a=10, b=1, c=9

but I've been unable to find out how to do this in the bourne shell.

Any help will be greatly appreciated.  

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