Hi @amailuk,
In the ODK Form Logic, unanswered number questions are nil. That is, they have no value. When a variable referencing an empty value is used in a math operator or function, it is treated as Not a Number (NaN). The empty value will not be converted to zero. The result of a calculation including NaN will also be NaN, which may not be the behavior you want or expect.
To convert empty values to zero, you can use either the coalesce() function or the if() function.
coalesce(${potentially_empty_value}, 0)
if(${potentially_empty_value}=“”, 0, ${potentially_empty_value})
