The Wall of a House Consists of a Slab of Concrete

The wall of a house consists of a slab of concrete (of R-value 0.5) with an insulating board (or R-value 1.5) firmly attached to the inside. The concrete and board are in thermal contact with each other. The temperature inside the house is 20 $\degree$ C and outside the house is 0 $\degree$ C. Determine the rate at which heat is lost through one square metre of wall (in W) .

A) 20
B) 10
C) 13.3
D) 40
E) 2.0