=> 300 hPa corresponds to FL300 - see note at the end of explanation.

In the standard troposphere, air temperature on average decreases with height at an overall positive lapse rate of about 2ºC/1000 ft, until the tropopause is reached. Once the tropopause is reached, temperature remains constant.

In this case, tropopause is said to be at FL330 - which means that from this level upwards, temperature remains constant. Therefore, temperature at FL330 = temperature at FL350.
In order to solve this exercise, we must find out the tropopause temperature (= temperature at FL330).
If the temperature at FL300 is -48ºC, we know that at FL330 it will be 6ºC colder => ((30 000 - 33 000)/1000) x 2 = -6ºC

• As discussed above, in the troposphere under normal circumstances, temperature decreases with increasing altitude => temperature at FL330 (and therefore, at FL350) = -48ºC - 6ºC = -54ºC.

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For the high-level charts, each FL corresponds to a specific pressure surface (pressure value):
FL50 = 850 hPa
FL100 = 700 hPa
FL140 = 600 hPa
FL180 = 500 hPa
FL240 = 400 hPa
FL300 = 300 hPa
FL340 = 250 hPa
FL390 = 200 hPa
FL450 = 150 hPa