.
The last string.
To get the correct answer, you need simply READ the problem.
Also, you would benefited, if you understand each word that you read.
It is ENOUGH to solve it and to answer correctly.
I think that you will learn NOTHING (well, not much, to be polite) solving such problems.
It is the same as if the teacher will draw one red square and one black square and will ask you which is black.
This exercise is to check if you can read and understand the meaning of what you read.