This is the boss. No calculator — today $g = 10\ \mathrm{N/kg}$, the round number every engineer keeps for site sanity checks. It makes $\rho g$ exactly $10\ \mathrm{kN/m^3}$ for water, so one kilopascal is a tenth of a metre and one litre per second lifted one metre costs ten watts. Every number below is pinned to fit in your head.

One pump room, and every answer feeds the next: the riser pressure becomes metres, the metres join the lift and the friction to make the total dynamic head, the head and the flow make the hydraulic power, the efficiency turns that into shaft power, and the shaft power decides the order. Write each line down — one early slip rolls all the way to the requisition.

Note what the last line actually asks. It does not want another number; it wants a decision, the way a real selection does. The arithmetic is only there to earn you the right to make it.

Then a bonus pair on the drive, where the exponents come home: flow once, power cubed. Carry $0.8^2 = 0.64$ and $0.8^3 = 0.512$ in your head and the whole chapter fits on the back of a work order. 5★ under par here is the chapter crown. Clock in.