The answer is 19.
I know the OP said not to explain, but I will explain my answer. The first line is correct, but not relevant. The other two lines are incorrect and still not relevant. The last line is the only one looking for an answer to the the equation which is 19.
What you stated is somewhat correct.
What we are given in the example is a 'mathematical sequence'. Whether the numbers actually are added correctly, in the base 10 system that we normally use, is irrelevant.
The example is asking for an answer to the sequence, as given.
