WHY must the same operation be performed on both sides of an equation?

Astore sells 4 apples for $3 and 3 oranges for $4 if pete buys 12 apples and 12 oranges how much will it cost

Me i’m stuck on all these questions besides the two bottom ones

The westins and the shermans live in the same city and pay the same sales tax rate, and both families made $14,000 in taxable purchases last year. if the westins made $86,000 and the shermans made $33,000 last year, is the sales tax in their city an example of a regressive tax? yes, because the westins and the shermans both paid the same sales tax rate. b. no, because the shermans paid a higher percentage of their income in sales tax than the westins did. c. no, because the westins and the shermans both paid the same sales tax rate. d. yes, because the shermans paid a higher percentage of their income in sales tax than the westins did.