64. We divide both sides of Eq. 19-32 by area A, which gives us the (uniform) rate of heat conduction per
unit area:
P
cond
A
= k
1
T
H
− T
1
L
1
= k
4
T
− T
C
L
4
where T
H
= 30
◦
C, T
1
= 25
◦
C and T
C
=
−10
◦
C. We solve for the unknown T .
T = T
C
+
k
1
L
4
k
4
L
1
(T
H
− T
1
) =
−4.2
◦
C .