Naive Ruby implementation of checksum algorithm

credit_naive.rb

def credit_card_check() #main ruby method
  credit_card_no = 0
  loop do #get credit card number
    puts "Number: "
    credit_card_no = gets.chomp
    size = credit_card_no.to_s.size
    break if size >= 13 && size <= 16
  end
  cardValid = checkCredit(credit_card_no)
  if cardValid == false
    puts "INVALID"
    return
  elsif credit_card_no.to_s[0] == "5"
    puts "MASTERCARD"
  elsif credit_card_no.to_s[0] == "4"
    puts "VISA"
  elsif credit_card_no.to_s[0] == "3"
    puts "AMEX"
  end
end

def checkCredit(card_no) #helper_method that implements checksum
  evenSum = 0
  odd = []
  card_no.to_s.reverse.split("").each_with_index do |digit, idx|
    if idx == 0 || idx % 2 == 0
      evenSum += digit.to_i
    else
      odd.push(digit.to_i * 2)
    end
  end

  oddSum = sumDigits(odd)
  #puts "Odd Sum: #{oddSum}, Even Sum: #{evenSum}"
  if (oddSum + evenSum) % 10 == 0
    return true
  else
    return false
  end
end

def sumDigits(arr) #helper_method that adds the digits of numbers in an array
  sum = 0
  arr.each do |number|
    if number > 9
      sum += number % 10
      sum += 1
    else
      sum += number
    end
  end
  return sum
end