solved 3.15
Merge branch 'master' of github.com:dtak1114/ProgramInOcamlPractices

* 'master' of github.com:dtak1114/ProgramInOcamlPractices:
  created empty file
  solved 4-6
  solved 4-5
  solvel 4-4
  removed q_13
  solved 4-3
  solved 4-1,4-2