Responsive Design Math

Calculating the responsive width of an element.
target-element / parent-of target-element = result

So you might have a target-element with a width = 420px
and the parent-of-target-element width = 660px

420px ÷ 660px = .63636363

.63636363 * 100 = 63.63636363

responsive-target-element {
  width: 63.63636363%;
}