Calculate Duration with Time in text-fields

Calculate Duration with Time in text-fields

Trying to subtract to decimals and get a time in minuets and seconds.

When trying to add the formula  Duration_in_Decimal - Duration_in_Decimal.toLong() * 60  to a text field I am getting error  :   Error at line number : 4   
Variable 'Single_Line' is already defined of data type 'STRING' but trying to update 'DECIMAL' data type