塔頂上有人 Voices On The Pagoda Top
又掃了三層，腰酸腿痛，就於十層上坐倒道：「悟空，你替我把那三層掃淨下來罷。」行者抖擻精神，登上第十一層，霎時又上到第十二層。正掃處，只聽得塔頂上有人言語。又扫了三层，腰酸腿痛，就于十层上坐倒道：「悟空，你替我把那三层扫净下来罢。」行者抖擞精神，登上第十一层，霎时又上到第十二层。正扫处，只听得塔顶上有人言语。
逐句解 Line by line
He swept three storeys more; his back ached and his legs hurt, and he sat down on the tenth, saying, "Wukong, sweep those three clean for me and come down."
坐倒 takes dǎo, the toppling reading: he does not so much sit as give way. 腰酸腿痛 is four characters of plain physical complaint, and it is the reason the vow finally gets handed over. 下來罷 ends on the soft ba again.
Pilgrim shook up his spirits, climbed to the eleventh storey, and in a moment was on the twelfth.
抖擻精神 is shaking yourself awake, rousing yourself, and the contrast with 腰酸腿痛 in the sentence before is the whole characterisation of the pair. Ten storeys took Tripitaka most of the night; two take Pilgrim a 霎時, an instant.
Just as he was sweeping, he heard voices on the top of the pagoda.
正掃處 uses 處 as chù, a place borrowed to mean a moment — at the point of sweeping. 只聽得 takes the full dé: the hearing comes off.
The arithmetic is exact and the novel expects you to follow it. Tripitaka sweeps seven storeys, then three more, and stops at the tenth; Pilgrim takes the eleventh and the twelfth; the thirteenth is where the voices are. The chapter has been counting since the question was asked.
It also matters that the discovery is made by the one who was not under the vow. Tripitaka undertook the sweeping and could not finish it. Pilgrim took over as a favour and walked into the answer on the second storey he swept.