[O] How to calculate sum of property in tree?

[Rainer Hansen]

Hi,

I would like Emacs to calculate the price for the travel cost
(21.90). This should be calculated as sum of the values of the property
"Price" in the elements of the tree. How do I do that in Emacs Lisp?

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* Travel cost -- 21.90 EUR

** Train Eusirchen -- Bonn, src_emacs-lisp{(org-entry-get (point) "Price")} EUR 
   :PROPERTIES:
   :Price:    7.70
   :END:
** Bus Euskirchen -- Stotzheim, src_emacs-lisp{(org-entry-get (point) "Price")} 
EUR
   :PROPERTIES:
   :Price:    2.40
   :END:

** Taxi Bonn, src_emacs-lisp{(org-entry-get (point) "Price")} EUR
   :PROPERTIES:
   :Price:    11.80
   :END:
----------------------------------------------

Thanks!

Rainer