A slab of concrete and an insulating board are in thermal contact with each other. The temperatures of their outer surfaces are 68°F and 50°F. Determine the rate of heat transfer (in BTU/ft2?h) if the R values are 1.93 and 8.7 ft2?°F?h/BTU, respectively.
A. 9.7
B. 2.5
C. 5.3
D. 1.7
E. 28
Answer: D
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