diff --git a/lib/blackjack_score.rb b/lib/blackjack_score.rb
index 18110b9..c0770a5 100644
--- a/lib/blackjack_score.rb
+++ b/lib/blackjack_score.rb
@@ -1,7 +1,42 @@
 # blackjack_score.rb
-
-VALID_CARDS = [1, 2, 3, 4, 5, 6, 7, 8, 9, 'King', 'Queen', 'Jack']
+require "pry"
+VALID_CARDS = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 'King', 'Queen', 'Jack']
 
 def blackjack_score(hand)
-
+  # Raise ArgumentError for invalid cards
+  if !hand.is_a?(Array)
+    raise ArgumentError, "Hand must be an array"
+  elsif hand.size > 3 || hand.size < 1
+    raise ArgumentError, "Hand must be 1-3 cards"
+  else
+    hand.each do |card|
+      if !VALID_CARDS.include?(card)
+        raise ArgumentError, "Invalid card(s)"
+      end
+    end
+  end
+  # Generates parallel array for score, e.g. [10, 1, 3]
+  scores_low_ace = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 10, 10]
+  card_to_score_low_ace = Hash[VALID_CARDS.zip(scores_low_ace)] # {1=>1, 2=>2, 3=>3, 4=>4, 5=>5, 6=>6, 7=>7, 8=>8, 9=>9, "King"=>10, "Queen"=>10, "Jack"=>10}
+  scores = []
+  hand.size.times do |i|
+    sum_before_this_card = scores.reduce(0, :+)
+    # ace default score is 1
+    # if card is an ace, score can be 11 in this case ONLY:
+    if hand[i] == 1 && sum_before_this_card < 10
+        scores << 11
+    else
+      # if card is not an ace
+      card_score = card_to_score_low_ace[hand[i]]
+      scores << card_score
+    end
+  end
+  # Sum
+  score = scores.reduce(:+)
+  # raise ArgumentError for scores over 21
+  if score > 21
+    raise ArgumentError, "You lose! Score > 21"
+  end
+  # return score
+  return score
 end
diff --git a/specs/blackjack_score_spec.rb b/specs/blackjack_score_spec.rb
index 6ef084a..9f9ec77 100644
--- a/specs/blackjack_score_spec.rb
+++ b/specs/blackjack_score_spec.rb
@@ -1,43 +1,84 @@
+#blackjack_score_spec.rb
+
 require 'minitest'
 require 'minitest/spec'
 require 'minitest/autorun'
 require 'minitest/reporters'
 require 'minitest/pride'
+require "minitest/skip_dsl"
 
 require_relative '../lib/blackjack_score'
 
 Minitest::Reporters.use! Minitest::Reporters::SpecReporter.new
 
-describe 'Blackjac Score' do
+describe 'Blackjack Score' do
   it 'can calculate the score for a pair of number cards' do
-
     # Arrange
-    hand = [3, 4]
-
+    hand = [6, 5]
     # Act
     score = blackjack_score(hand)
-
     # Assert <-  You do this part!
-
+    expect(score).must_equal 11
   end
 
   it 'facecards have values calculated correctly' do
-
+    face_cards = ["Jack", "Queen", "King"]
+    face_cards.each do |card|
+    # Arrange
+      hand = Array.new
+      2.times {hand.push(card)}
+    # Act
+      score = blackjack_score(hand)
+    # Assert <-  You do this part!
+      expect(score).must_equal 20
+    end
   end
 
   it 'calculates aces as 11 where it does not go over 21' do
-
+    # Arrange
+    hand = [9,1]
+    # Act
+    score = blackjack_score(hand)
+    # Assert <-  You do this part!
+    expect(score).must_equal 20
   end
 
   it 'calculates aces as 1, if an 11 would cause the score to go over 21' do
+    # Arrange
+    hand = [10, 10, 1]
+    # Act
+    score = blackjack_score(hand)
+    # Assert <-  You do this part!
+    expect(score).must_equal 21
 
+    # Arrange
+    hand = [10,1]
+    # Act
+    score = blackjack_score(hand)
+    # Assert <-  You do this part!
+    expect(score).must_equal 11
   end
 
   it 'raises an ArgumentError for invalid cards' do
-
+    # ARRANGE
+    possible_hands = [[10,1],[6,5]]
+    # Act-Assert
+    possible_hands.each do |hand|
+      score = blackjack_score(hand)
+      expect(score).must_equal 11
+    end
+    # Re-arrange
+    possible_hands = [[], nil, [0], ["Joker"], [1,2,3,4]]
+    # Act-Assert
+    possible_hands.each do |hand|
+      expect{blackjack_score(hand)}.must_raise ArgumentError
+    end
   end
 
   it 'raises an ArgumentError for scores over 21' do
-
+    # ARRANGE
+    hand = [10,10,2]
+    # ACT-ASSERT
+    expect{blackjack_score(hand)}.must_raise ArgumentError
   end
 end