What is disjunction calculation

In the calculation problem, there is a kind of problem that needs to use the recursive equation to calculate. Each line of calculation must be preceded by the equal sign and the result
format
99+1-35*2
　　=100-70
　　=30
Operation rules
It must be operated from left to right according to the rule of "multiplication and division before addition and subtraction". At the same time, the numbers in brackets, brackets and braces should be calculated first. Various operation rules can be used to make the calculation more simple and accurate. (Note: the formula can contain letters.)

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