Add a TIL on summing the values in collections.

ruby/summing-collections.md

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+# Summing Collections
+
+Given a hash, `my_hash`,
+
+```ruby
+my_hash = {
+    "one" => 1,
+    "two" => 2,
+    "three" => 5
+}
+```
+
+I want to determine the sum of the values in `my_hash`, which should be `8`.
+
+This is similar to asking for the sum of the values in an array. In fact,
+using `#values` on `my_hash` will leave us with the task of summing the
+values in an array.
+
+It turns out that to sum the values in an array, all you need is `#inject`
+
+```ruby
+[1,2,5].inject(:+)
+=> 8
+```
+
+So, if I want the sum of the values in a hash, such as `my_hash`, then all
+that is needed is
+
+```ruby
+my_hash.values.inject(:+)
+=> 8
+```