1. Pick any number between 70 and 979:
2. Add it to 1979:
3. Remove the thousands digit and add it to the remaining number:
4. Now subtract your last result from your original number:
5. Your answer is 19.

Here is an example:
1. Pick any number between 70 and 979:
      Pick 840.
2. Add it to 1979:
      840 + 1979 = 2819
3. Remove the thousands digit and add it to the remaining number:
      819 + 2 = 821
4. Now subtract your last result from your original number:
     840 - 821 = 19
5. Your answer is 19.