Yes, you can use bash's built-in Arithmetic Expansion $(( )) to do some simple maths
$ echo "$((5 * 5))"
Check the Shell Arithmetic section in the Bash Reference Manual for a complete list of operators.
For sake of completeness, as other pointed out, if you need arbitrary precision, bc or dc would be better.
~ Answered on 2012-06-14 19:20:09