ou are used to this by now. Write for me three methods - calculate, add and subtract. The tests should all pass. Take a look at the hint if you have trouble! And as a little extra hint: remember that you can use something.is_a?(Hash) or another_thing.is_a?(String) to check an object's type.

rubymonk.rb

def add(*numbers)
  numbers.inject(0) { |sum, number| sum + number }  
end

def subtract(*numbers)
  sum = numbers.shift
  numbers.inject(sum) { |sum, number| sum - number }  
end

def calculate(*arguments)
  # if the last argument is a Hash, extract it 
  # otherwise create an empty Hash
  options = arguments[-1].is_a?(Hash) ? arguments.pop : {}
  options[:add] = true if options.empty?
  return add(*arguments) if options[:add]
  return subtract(*arguments) if options[:subtract]
end