Single responsibility principle : a Kata

Some time ago I was browsing on http://rubyquiz.com/ and found a really interesting exercise there. It’s name is Getting to 100. The rules are quite simple:

  1. You are given the sequence of numbers 123456789
  2. Each number in the interval [1-9] has to be present in the sequence
  3. Each number must appear once and only once
  4. Numbers should be in increasing order
  5. You are given three operators: two minuses and one plus that you have to insert between those numbers
  6. The resulting equation should be give 100

The actual task is to: