My Swift / Xcode app does not perform a simple calculation : [Xcode screenshot ][1]


  [1]: https://i.stack.imgur.com/2Gzdj.png

 I have a simple app called BillSplitter that calculates a bill to display: input text fields for number of diners, bill total, and tip... 
when I run the app the grandTotal output does not calculate properly... the total and the tip do not calculate. Thanks for any help.