Final answer:
The calculation of the friction factor involves determining the Reynolds number and using the Moody diagram or an explicit equation, like eq. 7.6.26, which requires knowledge of the flow conditions, fluid properties, and pipe characteristics.
Step-by-step explanation:
The correct answer to determining the friction factor for water flowing in an 8-cm-diameter plastic pipe with a flow rate of 20 L/s at 20°C involves understanding hydrodynamics and requires the use of either the Moody diagram or a given equation (such as eq. 7.6.26, which the question seems to refer to).
The first step is to calculate the Reynolds number (ℝ) based upon the flow conditions. The Reynolds number is a dimensionless quantity used to predict flow patterns in different fluid flow situations. Once ℝ is determined, the Moody diagram can be used to find the corresponding friction factor if the surface roughness of the pipe is known.
Alternatively, using eq. 7.6.26 (which is not provided in the question, but is likely an explicit equation for the friction factor in turbulent or laminar flow conditions), one can directly calculate the friction factor based on the flow rate, viscosity of the fluid, pipe diameter, and potentially the roughness of the pipe's interior if the flow is turbulent.
Without the actual equation or the Moody diagram provided, a specific numerical answer cannot be calculated in this case. Moreover, this question requires an understanding of fluid mechanics, a core topic in mechanical and civil engineering.