Final answer:
Amanda experienced a temperature increase of 20 degrees from the original -8 degrees. By adding 20 to -8, we find that the temperature in Lisbon was 12 degrees when she arrived.
Step-by-step explanation:
When Amanda left her house in New York, it was -8 degrees. After her plane landed in Lisbon and she discovered it was 20 degrees hotter, to find out the new temperature we simply add the increase in temperature to the original temperature.
So, starting at -8 degrees and increasing by 20 degrees:
-8 + 20 = 12 degrees.
Therefore, the temperature when Amanda landed in Lisbon was 12 degrees.