my working solution/spec pair

lib/fizzbuzz.rb

@@ -1,3 +1,4 @@
+#one line!
 def fizzbuzz(num)
-    'fizz'
+   return((num % 15).zero? ? "fizzbuzz" : (num % 3).zero? ? "fizz" : (num % 5).zero? ? "buzz" : num)
 end

spec/fizzbuzz_spec.rb

@@ -1,7 +1,16 @@
-require ./lib/'fizzbuzz'
+require './lib/fizzbuzz'
 
 describe 'fizzbuzz' do
-    it  'returns "fizz" when passed 3' do
+    it  'returns "fizzbuzz" when passed a multiple of 3 and 5' do
+        expect(fizzbuzz(15)).to eq 'fizzbuzz'
+    end
+    it  'returns "fizz" when passed a multiple of 3' do
         expect(fizzbuzz(3)).to eq 'fizz'
     end
+    it  'returns "buzz" when passed a multiple of 5' do
+        expect(fizzbuzz(5)).to eq 'buzz'
+    end
+    it  'returns echoes when passed none of the above' do
+        expect(fizzbuzz(4)).to eq 4
+    end
 end