Answer:
2 C1 = 4 C2 heat loss = heat gain
C1 = 2 C2
17.9 C2 = 3.1 C3 mixing second and third
17.9 / 2 C1 = 3.1 C3 C1 for C2
17.9 C1 = 6.2 C3
C3 = 2.89 C1
C1 (T - 13) = C3 (40 - T) heat loss = heat gain
T - 13 = 2.89 (40 - T) = 115.5 - 2.89 T
3.89 T = 128.5
T = 33 deg C