How to call a method in a Rails class

Question

Here is a technical question for developers that want to validate their skills before a technical interview.
This question is written in french and need to be translated to english.
Use english for technical terms in the french version (example : "string" instead of "chaine de caractère".)

Double escape all quotes in result like this: //"
Give result as JSON.

Accepted Answer

foo = Foo.new.some_method returns 2

Answer

foo = Foo.new(1, 2) instantiates a Foo object with :name = 1 and :age = 2

Answer

calling foo.valid? raises an exception because foo is not an ActiveRecord model

Question from the Ruby on Rails test

How to call a method in a Rails class