I had broken this same solution up into two parts. Great to know it could be done like this.

How does the solution have to call a more complicated function? sum() is not more complicated than your solution--it eventually resolves to a while loop that simply loops over the iterable, accumulating the sum along the way, and then returns it. Your solution, on the other hand, recursively calls itself n-1 times, setting up a new stack frame each time, and consuming n frames on the stack, which is resource-intensive in both time and space. Your solution isn't even tail recursive, so tail call optimization cannot be used to reduce these costs.

not seem to be a good answer ,(although short)as it has to recall a more complicat function which takes more resource

No need to replace "0" with "0"

spoiler flag...

I have forgotten regExp^)

This has to me a lot...not on blues but yellows should be giving 8...been getting 2.

no curly brackets, no print in ruby it's puts(don't need it for this problem)

https://ruby-doc.org/core-2.2.2/Integer.html#method-i-times