calculator.rb

# Functions for preforming calculations

def addition(first_num, second_num)
  puts "#{first_num} + #{second_num} = "
  puts first_num.to_i + second_num.to_i
end

def subtraction(first_num, second_num)
  puts "#{first_num} - #{second_num} = "
  puts first_num.to_i - second_num.to_i
end

def multiplication(first_num, second_num)
  puts "#{first_num} * #{second_num} = "
  puts first_num.to_i * second_num.to_i
end

def division(first_num, second_num)
  puts "#{first_num} / #{second_num} = "
  puts first_num.to_i / second_num.to_i
end

def modulo(first_num, second_num)
  puts "#{first_num} % #{second_num} = "
  puts first_num.to_i & second_num.to_i
end


puts "Let's do some math!\n\nLet's start with a number"
first_num = gets.chomp
first_num_check = first_num.to_i.to_s
while first_num_check != first_num do
  puts "Oops! This calculator takes numbers only! Try again:"
  print ">> "
  first_num = gets.chomp
  first_num_check = first_num.to_i.to_s
end
puts "you entered => #{first_num}"
puts "What what would you like to do to #{first_num}?"
operator = gets.chomp.downcase
operator_check = %w(addition subtraction multiplication modulo division add subtract multiply mod divide + - * % /)
while operator_check.include?(operator) == false do
  puts "Oops! I don't know how to do that!"
  puts "Try one of these operators: #{operator_check[operator_check.length-operator_check.length/3...operator_check.length].join(", ")}"
  print ">> "
  operator = gets.chomp.downcase
end
puts "OK! We've got #{first_num} #{operator} by what number?"
print ">> "
second_num = gets.chomp
second_num_check = second_num.to_i.to_s
while second_num != second_num_check do
  puts "Oops! This calculator takes numbers only! Try again:"
  print ">> "
  second_num = gets.chomp
  second_num_check = second_num.to_i.to_s
end

case operator
  when "additon", "add", "+"
    puts addition(first_num, second_num)
  when "subtraction", "subtract", "-"
    puts subtraction(first_num, second_num)
  when "multiplication", "multiply", "*"
    puts multiplication(first_num, second_num)
  when "division", "divide", "/"
    puts division(first_num, second_num)
  when "modulo", "mod", "%"
    puts modulo(first_num, second_num)
end

Nice use of the case statement to handle your operations! I like the way the case statement blocks use methods too! nice!