Six variables are provided for you to store values while the calculator is running: a, b, c, x, y, z.
To store a value to a variable you just need to assign it in the expression field, e.g.:
a := 10
or
b := 5
To retrieve the value of a variable type its name in the expression field and press Calculate, e.g.:
a
You can also use the variable in subsquent expressions, e.g.:
a + b