Question

何以「四则运算」一定要「先乘除，后加减」?

Answer 1

无所谓甚么原理
而是自四则运算运算制订时订立的原则. "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." "四则运算（Four Arithmetic Operations），即加减乘除，是数学最基本的运算。如果加减乘除放在同一个算式列中的话，其计算的顺序是，先乘除，后加减，括号先算。四则运算的起源很早，几乎在数学产生时就有了。"
无所谓甚么原理
而是自四则运算运算制订时订立的原则. "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." "四则运算（Four Arithmetic Operations），即加减乘除，是数学最基本的运算。如果加减乘除放在同一个算式列中的话，其计算的顺序是，先乘除，后加减，括号先算。四则运算的起源很早，几乎在数学产生时就有了。"