Refactoring rpn interview solution

rpn.rb

def evaluate_rpn(string)
OPERATORS = ["+", "-", "*", "/"]
chars = string.split(" ")
numbers = []
chars.each do |char|
    if !OPERATORS.include?(char)
        numbers << char
    else 
        second = numbers.pop
        first = numbers.pop
        string = "#{first}#{char}#{second}"
        output = eval string
        numbers << output.to_s
    end
end
numbers.first
end

Instead of building a string like this:

string += first
string += char
string += second

we can just use string interpolation