This is the boss. No calculator. Today the enthalpy runs on the site-sanity form $h \approx t + 2.5\,W$, with $t$ in °C and $W$ in g/kg — 1.006 rounded to 1, and $(2501 + 1.86\,t)$ rounded to 2 500 J per gram. It sits within about 1 % of the full relation across the comfort range, which is closer than anyone reads a chart, and it is the arithmetic a good engineer does standing in front of the unit.

One air handler, one long problem, and every answer feeds the next: the mixed temperature, then the mixed moisture on the same weights, then the enthalpy those two make, then the load the coil is charged for. Write each line down as you go — one early slip rolls all the way to the nameplate.

And note what the last line actually asks. It does not want another number; it wants a DECISION — does the coil that is already bolted in there carry this load. The arithmetic is only there to earn you the right to make it.

Everything in the chapter rides in this one unit. 5★ under par here is the chapter crown. Clock in.