Ruby on Rails

Being new to rails and ruby I'm a bit lost.

Have a data entry scaffold, Details, which has name, string,
cost1:integer cost2:integer cost3: Total:integer
The user inputs puts a value into each of 3 fields.

The totals field should ideally be automatically updated as each value
is entered or changed. Then Save stores all 4 values into the database.
One of the values cost3 should display a tax value of 10% of that
particular entry beside the entry.

Is this possible to do this or should I have something like a cart and
display the calculations in that?

I have read the Agile Web Development books, Enterprise Recipes,
Advanced Rails, Why's Poignant, Head First Rails which is for rails2 so
some scripts fail.
None explain how to do some of the more simple things like the above or
taking a value from a select list and using it to update a field in
another view.
I've been searching for days with no luck,
Thank you in advance
R

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