diff --git a/lib/binary_to_decimal.rb b/lib/binary_to_decimal.rb
index 439e8c6..9e8ef50 100644
--- a/lib/binary_to_decimal.rb
+++ b/lib/binary_to_decimal.rb
@@ -5,5 +5,13 @@
 # Calculate  and return the decimal value for this binary number using
 # the algorithm you devised in class.
 def binary_to_decimal(binary_array)
-  raise NotImplementedError
+
+  # without each_with_index
+  sum = 0
+  index = 0
+  binary_array.each do |digit|
+    sum += digit * ( 2 ** ( 7 - index))
+    index += 1
+  end
+  return sum
 end