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.