Average temperature

The average temperature of six cities is 5$^\circ$C. What is the average temperature when two cities are added?
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The average temperature in 6 European cities is 5$^\circ$ Celsius.

What is the new average temperature, if two more cities, with temperatures of 12$^\circ$ Celsius and $-$6$^\circ$ Celsius, are added to the list?

This problem is adapted from the World Mathematics Championships