Peter,
The basic principle for the TextInput object is to assign a Page variable to it.
(you can use Publication variable as well)
See that under Properties >>> Text Input Tab (see attached)
Several options there:
-- configure for numbers only entry
-- confine to one line, or a limited number of characters
-- various options for when to update the variable
-- and of course the variable assignment itself ( you can select from dropdown, or Click "New" to create on)
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Here is an extension of that approach.
Position your TextInput boxes off to the side, and where you want to illustrate the calculation, put a Text object. In that last object, you "Insert Variable" and select the var that you update with TextInput.
The result is to input in one area and show updated calculation in other area of page.
similar reference here:
viewtopic.php?f=6&t=3542 (you just won't need the 'drag&drop' there)
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One last advice. Make sure you do not use the same name for an Object and a Variable. That is a 'no-no', gives unpredictable results.