Pre-algebra simultaneous equations. Clever!
盈不足術
/yíng bù zú shù/
rule of too much and not enough (!)
حساب الخطأين
/ḥisāb al-khaṭa'ayn/
reckoning from two falsehoods
regula duorum falsorum
rule of double false position
Subtract if both guesses are deficits/excesses.
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