During a mathematics activity, four students stood on consecutive floors of a building: 10, 11, 12 and 13. Their teacher asked them to perform different calculations by placing '\(+\)' and '\(−\)' signs between the floor numbers. The students noticed that although the values changed, the results always remained even. They realised that changing the sign of a number changes the value but does not change the parity of the final result.
1. What is the sum of all four floor numbers?
2. Which of the following expressions gives the value 20?
3. The result obtained in question (2) is
4. Why does the parity remain unchanged?