#### description

- if we add the same number to both sides of an equality, it still holds,
- if we subtract the same number from both sides of an equality, it still holds,
- if we multiply or divide both sides of the equality by the same non-zero number, it still holds.
- if we fail to do the same mathematical operation on both sides of an equality, the equality does not hold.
- In each equation variable represents a number only.