Answer:
1. Add zeros to get same amount of zeros after comma (to get easier addition):
2.31200
0.00045
—————
2.31245
So the right answer is 2.31245, but misconception in this operation, is that people think zeros have no value, so you can skip them, so people add 2.312 to 0.00045, like it was 2.312 + 0.045, and this is 2.357.