Oslo and Paris are marked on the chart and you can work out the average temperature by adding the up and dividing by the number you have added:

            (-25ºC) + (-24ºC) + (-23ºC) + (-21ºC) + (-21ºC) + (-20ºC) = -134º ÷ 6 = -22.3ºC

This is the temperature at FL180 and at a typical lapse rate of 2ºC per 1,000ft the temperature at FL160 will be about 4ºC warmer at approximately -18ºC.