Systematic Treatise on Arithmetic

Add seventy to the number to get eighty-seven. Two jin A jin is a unit of weight, roughly 600 grams. serves as the son's number; perform the calculation this way, adding seven by seven likely a shorthand or error for "seventeen," which was the difference in the previous problem until reaching the eldest to match the requirements of the question.

Song of Asking the Ages of Nine Sons

An old father has nine sons,
If asked their birth years, they are generally unknown.
Arranged in order from the eldest, they differ by three years;
Their total years sum to a period of two hundred and seven.
May I ask how old the eldest son is?
The age of each son must be carefully deduced.

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Song of Asking the Ages of Nine Sons original: 九兒問甲歌 (Jiǔ ér wèn jiǎ gē). Jia here refers to one's place in the cycle of years, or age.; Father original: 公公 (gōnggōng), a respectful address for an elderly patriarch.; Age original: 歲數 (suìshù), refers to the traditional Chinese method of counting years of age.; Detailed deduction original: 詳推 (xiángtuī), to calculate or infer step-by-step.; Arithmetic original: 算術 (suànshù), the general study of numbers and calculation.; Arithmetic progression original: 等差數列 (děngchā shùliè), a sequence where the difference between consecutive terms is constant.