A stratified confined aquifer consists of three parallel homogeneous and isotropic horizontal layers with thickness of 10 m, 5 m, and 5 m. The layers have the same width. The hydraulic conductivities of the strata are 15 m/day, 20 m/day, and 30 m/day, respectively. The water flow follows Darcy's law and is parallel to the strata. Considering the same hydraulic gradient for all the layers, the effective hydraulic conductivity of the aquifer is \(\underline{\hspace{1cm}}\) m/day.