You can use the following basic operators in your fields' display text or in the values of your skip or validation logic. 



OperatorDescription
+Adds two values or expressions together.
-Subtracts one value or expression from another.
*Multiplies two values or expressions together.
/Divides one value or expression by another.
modFinds the remainder after division of one expression by another.
|Performs a union on two expressions.
andPerforms a boolean AND on two expressions.
orPerforms a boolean OR on two expressions.
 =Performs an 'equals to' comparison on two expressions.
!=Performs a 'not equals to' comparison on two expressions.
<=Performs a 'less than or equals to' comparison on two expressions.
<Performs a 'less than' comparison on two expressions.
>=Performs a 'greater than or equals to' comparison on two expressions.
>Performs 'greater than' comparison on two expressions.




Tip: if your calculation does not seem to work correctly, try adding white space around the operator, e.g. instead of "=($field_1+5)" use "=($field_1 + 5)"




Also see here for more functions and expressions you can use.