Methods calculate, add, subtract

CODE:
def calculate(*args)
  if args[-1].is_a?(Hash)
    if args[-1][:add]
      add(*args[0..-2])
    elsif args[-1][:subtract]
      subtract(*args[0..-2])
    end
  else
    add(*args)
  end
end

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

def subtract(*numbers)
  numbers.inject { |dif, num| dif - num }
end

TESTS from rubymonk

invoking add(4, 5) returns 9 ✔

invoking add(-10, 2, 3) returns -5 ✔

invoking add(0, 0, 0, 0) returns 0 ✔

invoking subtract(4, 5) returns -1 ✔

invoking subtract(-10, 2, 3) returns -15 ✔

invoking subtract(0, 0, 0, 0, -10) returns 10 ✔

defaults to addtion when no option is specified ✔

invoking calculate(4, 5, add: true) returns 9 ✔

invoking calculate(-10, 2, 3, add: true) returns -5 ✔

invoking calculate(0, 0, 0, 0, add: true) returns 0 ✔

invoking calculate(4, 5, subtract: true) returns -1 ✔

invoking calculate(-10, 2, 3, subtract: true) returns -15 ✔

invoking calculate(0, 0, 0, 0, -10, subtract: true) returns 10 ✔