Calculation is concatenating numbers instead of calculating them

This will occur if any of the number values are set to a blank value. When this is the case A2J Author® treats the variable as a string and starts to append the numbers instead of doing your calculation. To ensure that the values are numbers you can either require a value from the user and default the value to some acceptable number or after a button is pressed verify that each item has a number value and if not set it to a default value.