assuming that all the five-character poems are seven words, the five-character poems should have 7 * 13 = 91 words more than the seven-character poems, but actually there are 21 words less, with a difference of 91+21 = 112 words, because each sentence has a difference of 7-5 = 2 words, so the five-character poems with 112/2 = 56 sentences are assumed to be seven-character poems. That is, there are 56 sentences in the five-character poem.
five-character poem: (7 * 13+21)/(7-5)
= 112/2
= 56 (sentence)
seven-character poem: 56-13 = 43 (sentence)
test: 43 * 7-5 *.