diff --git a/ruby/phone-number/phone_number.rb b/ruby/phone-number/phone_number.rb
index d400971..831ee8e 100644
--- a/ruby/phone-number/phone_number.rb
+++ b/ruby/phone-number/phone_number.rb
@@ -1,13 +1,7 @@
 class PhoneNumber
   def self.clean(num)
-    cl = num.scan(/\d/).join
-    invalid?(num) ? nil : cl
-  end
-
-  def self.invalid?(num)
-    if num.start_with?("1")
-      num.size == 11
-    end
+    n = num.scan(/\d/).join.delete_prefix('1')
+    n.size == 10 && !n[3].match?(/[01]/) ? n : nil
   end
 end
 
diff --git a/ruby/prime-factors/README.md b/ruby/prime-factors/README.md
new file mode 100644
index 0000000..07ef0c4
--- /dev/null
+++ b/ruby/prime-factors/README.md
@@ -0,0 +1,60 @@
+# Prime Factors
+
+Compute the prime factors of a given natural number.
+
+A prime number is only evenly divisible by itself and 1.
+
+Note that 1 is not a prime number.
+
+## Example
+
+What are the prime factors of 60?
+
+- Our first divisor is 2. 2 goes into 60, leaving 30.
+- 2 goes into 30, leaving 15.
+  - 2 doesn't go cleanly into 15. So let's move on to our next divisor, 3.
+- 3 goes cleanly into 15, leaving 5.
+  - 3 does not go cleanly into 5. The next possible factor is 4.
+  - 4 does not go cleanly into 5. The next possible factor is 5.
+- 5 does go cleanly into 5.
+- We're left only with 1, so now, we're done.
+
+Our successful divisors in that computation represent the list of prime
+factors of 60: 2, 2, 3, and 5.
+
+You can check this yourself:
+
+- 2 * 2 * 3 * 5
+- = 4 * 15
+- = 60
+- Success!
+
+* * * *
+
+For installation and learning resources, refer to the
+[exercism help page](http://exercism.io/languages/ruby).
+
+For running the tests provided, you will need the Minitest gem. Open a
+terminal window and run the following command to install minitest:
+
+    gem install minitest
+
+If you would like color output, you can `require 'minitest/pride'` in
+the test file, or note the alternative instruction, below, for running
+the test file.
+
+Run the tests from the exercise directory using the following command:
+
+    ruby prime_factors_test.rb
+
+To include color from the command line:
+
+    ruby -r minitest/pride prime_factors_test.rb
+
+
+## Source
+
+The Prime Factors Kata by Uncle Bob [http://butunclebob.com/ArticleS.UncleBob.ThePrimeFactorsKata](http://butunclebob.com/ArticleS.UncleBob.ThePrimeFactorsKata)
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.
diff --git a/ruby/prime-factors/prime_factors.rb b/ruby/prime-factors/prime_factors.rb
new file mode 100644
index 0000000..bd2dafb
--- /dev/null
+++ b/ruby/prime-factors/prime_factors.rb
@@ -0,0 +1,15 @@
+class PrimeFactors
+  def self.for(i)
+    r = []
+    d = 2
+    while i > 1
+      if i % d == 0
+        i /= d
+        r << d
+      else
+        d += 1
+      end
+    end
+    r
+  end
+end
diff --git a/ruby/prime-factors/prime_factors_test.rb b/ruby/prime-factors/prime_factors_test.rb
new file mode 100644
index 0000000..c6e69d5
--- /dev/null
+++ b/ruby/prime-factors/prime_factors_test.rb
@@ -0,0 +1,48 @@
+require 'minitest/autorun'
+require_relative 'prime_factors'
+
+class PrimeFactorsTest < Minitest::Test
+  def test_1
+    assert_equal [], PrimeFactors.for(1)
+  end
+
+  def test_2
+    assert_equal [2], PrimeFactors.for(2)
+  end
+
+  def test_3
+    assert_equal [3], PrimeFactors.for(3)
+  end
+
+  def test_4
+    assert_equal [2, 2], PrimeFactors.for(4)
+  end
+
+  def test_6
+    assert_equal [2, 3], PrimeFactors.for(6)
+  end
+
+  def test_8
+    assert_equal [2, 2, 2], PrimeFactors.for(8)
+  end
+
+  def test_9
+    assert_equal [3, 3], PrimeFactors.for(9)
+  end
+
+  def test_27
+    assert_equal [3, 3, 3], PrimeFactors.for(27)
+  end
+
+  def test_625
+    assert_equal [5, 5, 5, 5], PrimeFactors.for(625)
+  end
+
+  def test_901255
+    assert_equal [5, 17, 23, 461], PrimeFactors.for(901_255)
+  end
+
+  def test_93819012551
+    assert_equal [11, 9539, 894_119], PrimeFactors.for(93_819_012_551)
+  end
+end
diff --git a/ruby/strain/README.md b/ruby/strain/README.md
new file mode 100644
index 0000000..4c4bb30
--- /dev/null
+++ b/ruby/strain/README.md
@@ -0,0 +1,64 @@
+# Strain
+
+Implement the `keep` and `discard` operation on collections. Given a collection
+and a predicate on the collection's elements, `keep` returns a new collection
+containing those elements where the predicate is true, while `discard` returns
+a new collection containing those elements where the predicate is false.
+
+For example, given the collection of numbers:
+
+- 1, 2, 3, 4, 5
+
+And the predicate:
+
+- is the number even?
+
+Then your keep operation should produce:
+
+- 2, 4
+
+While your discard operation should produce:
+
+- 1, 3, 5
+
+Note that the union of keep and discard is all the elements.
+
+The functions may be called `keep` and `discard`, or they may need different
+names in order to not clash with existing functions or concepts in your
+language.
+
+## Restrictions
+
+Keep your hands off that filter/reject/whatchamacallit functionality
+provided by your standard library!  Solve this one yourself using other
+basic tools instead.
+
+* * * *
+
+For installation and learning resources, refer to the
+[exercism help page](http://exercism.io/languages/ruby).
+
+For running the tests provided, you will need the Minitest gem. Open a
+terminal window and run the following command to install minitest:
+
+    gem install minitest
+
+If you would like color output, you can `require 'minitest/pride'` in
+the test file, or note the alternative instruction, below, for running
+the test file.
+
+Run the tests from the exercise directory using the following command:
+
+    ruby strain_test.rb
+
+To include color from the command line:
+
+    ruby -r minitest/pride strain_test.rb
+
+
+## Source
+
+Conversation with James Edward Gray II [https://twitter.com/jeg2](https://twitter.com/jeg2)
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.
diff --git a/ruby/strain/strain_test.rb b/ruby/strain/strain_test.rb
new file mode 100644
index 0000000..1e3e8c8
--- /dev/null
+++ b/ruby/strain/strain_test.rb
@@ -0,0 +1,87 @@
+require 'minitest/autorun'
+require_relative 'strain'
+
+class ArrayTest < Minitest::Test
+  def test_empty_keep
+    assert_equal [], [].keep { |e| e < 10 }
+  end
+
+  def test_keep_everything
+    skip
+    assert_equal [1, 2, 3], [1, 2, 3].keep { |e| e < 10 }
+  end
+
+  def test_keep_first_and_last
+    skip
+    assert_equal [1, 3], [1, 2, 3].keep(&:odd?)
+  end
+
+  def test_keep_neither_first_nor_last
+    skip
+    assert_equal [2, 4], [1, 2, 3, 4, 5].keep(&:even?)
+  end
+
+  def test_keep_strings
+    skip
+    words = %w(apple zebra banana zombies cherimoya zelot)
+    result = words.keep { |word| word.start_with?('z') }
+    assert_equal %w(zebra zombies zelot), result
+  end
+
+  def test_keep_arrays
+    skip
+    rows = [
+      [1, 2, 3],
+      [5, 5, 5],
+      [5, 1, 2],
+      [2, 1, 2],
+      [1, 5, 2],
+      [2, 2, 1],
+      [1, 2, 5]
+    ]
+    result = rows.keep { |row| row.include?(5) }
+    assert_equal [[5, 5, 5], [5, 1, 2], [1, 5, 2], [1, 2, 5]], result
+  end
+
+  def test_empty_discard
+    skip
+    assert_equal [], [].discard { |e| e < 10 }
+  end
+
+  def test_discard_nothing
+    skip
+    assert_equal [1, 2, 3], [1, 2, 3].discard { |e| e > 10 }
+  end
+
+  def test_discard_first_and_last
+    skip
+    assert_equal [2], [1, 2, 3].discard(&:odd?)
+  end
+
+  def test_discard_neither_first_nor_last
+    skip
+    assert_equal [1, 3, 5], [1, 2, 3, 4, 5].discard(&:even?)
+  end
+
+  def test_discard_strings
+    skip
+    words = %w(apple zebra banana zombies cherimoya zelot)
+    result = words.discard { |word| word.start_with?('z') }
+    assert_equal %w(apple banana cherimoya), result
+  end
+
+  def test_discard_arrays
+    skip
+    rows = [
+      [1, 2, 3],
+      [5, 5, 5],
+      [5, 1, 2],
+      [2, 1, 2],
+      [1, 5, 2],
+      [2, 2, 1],
+      [1, 2, 5]
+    ]
+    result = rows.discard { |row| row.include?(5) }
+    assert_equal [[1, 2, 3], [2, 1, 2], [2, 2, 1]], result
+  end
+end