This is the boss. No calculator. Tonight the two awkward constants are handed to you rounded, and every line is integer arithmetic: on hard ground the 1 m spreading term is exactly $-8$ dB (the true figure is $-7.98$), and every doubling of distance costs exactly 6 dB (the true figure is 6.02). Criterion 85 dBA, exchange 3 dB.

One yard, and every answer feeds the next: a nameplate power to a level at 1 m, 1 m out to where the man actually stands, one machine to two, and the level to the clock. Write each line down as you go — one slip at the top rolls all the way to the verdict.

Note what the last line asks for. It does not want another number; it wants a decision: may this stint go ahead as planned. The arithmetic exists only to earn you the right to make it. Watch for the trap the whole chapter has been building — the second machine adds only 3 dB, which sounds like nothing, and on a 3 dB exchange it is exactly one halving of the permitted hours.

A bonus mark waits on the way out, at the garden fence behind the yard. One more constant is handed over for it: a source that holds the same level day and night has a day–night level exactly 6.4 dB above that level (the true figure is 6.41). The cap there is written in $L_{dn}$, so the meter reading alone settles nothing.

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