Pumps & Systems, Nov. 2016, by Lev Nelik.
http://www.pumpsandsystems.com/pumps/november-2016-another-look-optimizing-pump-hydraulics

"This column will provide feedback on a quiz published in my August 2016 Pumps & Systems column titled “Optimizing Pump Hydraulics: When Is It Important?” (Read it here) Readers were asked to estimate the amount of energy that would be saved by using a variable frequency drive (VFD) with a small centrifugal pump and to compare those savings to the energy used by a pump specifically designed for the required conditions (100 gallons per minute [gpm], 15 feet)."