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 .