(x - wx, x + wx) + (y - wy, y + wy) = (x + y - (wx + wy), x + y + (wx + wy))
=> width of the sum of intervals is the sum of the interval widths


(x - wx, x + wx) - (y - wy, y + wy) = (x - wx - y - wx, x + wx - y + wy)
                                    = (x - y - (wx + wy), x - y + (wx + wy))
=> width of the difference of intervals is the sum of the interval widths

The width of the product of intervals is not a function of the interval widths
ex:
For y > 0
(x - wx, x + wx) * (y, y) = (xy - (wx*y), xy + (wxy))
=> width is a function of the intervals - in addition to the widths

The same applies for division
(x - wx, x + wx) / (y, y) = (x/y - wx/y, wx + wx/y)
=> width depends on the intervals - in addition to the widths