Linux Shell Scripting Tutorial (LSST) v1.05r3  
Chapter 7: awk Revisited  

You can easily, do the arithmetic with awk as follows
$ cat > math 
Run the awk program as follows:
$ awk f math
20 3
20 + 3 = 23
20  3 = 17
20 / 3 = 6.66667
20 x 3 = 60
20 mod 3 = 2
(Press CTRL + D to terminate)
In above program print $1 " + " $2 " = " $1 + $2, statement is used for addition purpose. Here $1 + $2, means add (+) first field with second field. Same way you can do  (subtraction ), * (Multiplication), / (Division), % (modular use to find remainder of division operation).
Predefined variables of awk  User Defined variables in awk 