What is the correct method for solving thermal conductivity in this problem?

[Zhao_1911]

Homework Statement

One end of a metal rod is maintained at 100 degrees C, and the other end is maintained at 0 degrees C by an ice-water mixture. The rod is 60 cm long and has a cross-sectional area of 1.25 cm^2. The heat conducted by the rod melts 8.50 g of ice in 10.0 min. Find the thermal conductivity of the metal if 30% of heat is lost to the surroundings.

Homework Equations

(Q/t) = mLf/t
where (Q/t) is the heat flow, Lf is the latent heat of fusion

(Q/t) = KA(delta T)/L
where K is the thermal conductivity of the metal, A is the area, delta T is the temperature difference, and L is the length.

The Attempt at a Solution

First, I solved the heat flow of the metal using the mass of the melted ice and the time it took to melt that amount of ice;

(Q/t)= [8.5g(80cal/g)x.70]/600s = 119/150 cal/s

Then I solved the K of the metal;

119/150 cal/s = [K(1.25 cm^2)(100C-0C)]/60 cm
K = 0.381 cal/cm.s.C

BUT THEN..
I saw a solution of my classmate that used something like this;

(Q/t)={[8.5g(80cal/g)]/.70}/600s = 34/21 cal/s

so his K is about 0.777 cal/cm.s.C

that's where I'm confused. Should I divide or multiply the .70?

 

[LawrenceC]

Your friend is correct.