Could someone please tell me what I'm doing wrong? I'm getting decimal values, but the answers should be integers. I think I messed up in the equations.
My equations using Kirchoff's loop rule are:
12 - 500I1 - 815(I1-I3) = 0
12 - 269I2 - 439(I2 + I3) = 0
-500I1 + 269I2 = 0.
The printed question is the assignment, and the hand-drawn diagram is what I used to make the equations.

A47.2 g block of copper whose temperature is 480 k is placed in an insulating box with a 91.8 g block of lead whose temperature is 200 k. (a) what is the equilibrium temperature of the two-block system? (b) what is the change in the internal energy of the two-block system between the initial state and the equilibrium state? (c) what is the change in the entropy of the two-block system? the heat capacities of copper and lead are 386 j/kg·k and 128 j/kg·k, respectively.