Remember that the temperatures are in oK and the answers are ISA deviations.

ISA+10

TAT = SAT (1+ 0.2M²)
SAT = TAT / (1+ 0.2 Kr M²)

Kr is your recovery factor.

TAT is -5 C, which is 268.15 K. 268.15 / (1 + 0.2 x 0.84 x 0.36²) = 262.436 K ("SAT"), so 262.436 - 273.15 =

-10.7137 C ("SAT").

FL180..... 18 x 1.9812 = 35.6616 C of a drop, starting at 15 C, so in ISA is -20.6616. Therefore we're about 10 C warmer than ISA.