In an aquifer there are three horizontal layers A, B, and C on top of each other. A: Saturated horizontal hydraulic conductivity is 20 m/d and is 20 m thick B: Saturated horizontal hydraulic conductivity is 40 m/d and is 30 m thick C: Saturated horizontal hydraulic conductivity is 50 m/d and is 10 m thick The hydraulic gradient in the aquifer is horizontal and its magnitude is 0.001. What is the upscaled horizontal hydraulic conductivity of the aquifer? (m/d)