何以「四則運算」一定要「先乘除，後加減」?

無所謂甚麼原理, 而是自四則運算運算制訂時訂立的原則.
"In arithmetic and algebra, when a number or expression is both preceded and followed by a binary operation, a rule is required for which operation should be applied first. From the earliest use of mathematical notation, multiplication took precedence over addition, whichever side of a number it appeared on. Thus 3 + 4 • 5 = 5 • 4 + 3 = 23."
"四則運算（Four Arithmetic Operations），即加減乘除，是數學最基本的運算。如果加減乘除放在同一個算式列中的話，其計算的順序是，先乘除，後加減，括號先算。四則運算的起源很早，幾乎在數學產生時就有了。"

無所謂甚麼原理, 而是自四則運算運算制訂時訂立的原則.

"In arithmetic and algebra, when a number or expression is both preceded and followed by a binary operation, a rule is required for which operation should be applied first. From the earliest use of mathematical notation, multiplication took precedence over addition, whichever side of a number it appeared on. Thus 3 + 4 • 5 = 5 • 4 + 3 = 23."

"四則運算（Four Arithmetic Operations），即加減乘除，是數學最基本的運算。如果加減乘除放在同一個算式列中的話，其計算的順序是，先乘除，後加減，括號先算。四則運算的起源很早，幾乎在數學產生時就有了。"