Through hypothesis and experiments
The frequency of a wave represents B. the number of wave cycles that pass through a specific point within a given time.
The distance between two consecutive crests and the length of a wave are the <em>wavelength</em>.
The distance between the highest and lowest points of a wave is <em>twice the amplitude</em>.
When writing an ionic compound formula, a "molecular" form is used. The formula is made with allowance for ion charges.
For example,
Ca²⁺ and NO₃⁻ ⇒ Ca(NO₃)₂
Al³⁺ and SO₄²⁻ ⇒ Al₂(SO₄)₃
4 x 1000 so 4 kilometers are equal to 4000 meters