How does price changes affect the demand for goods that are unrelated?

Well, if the amount of a object increased, but there was a alternative you could buy that would save you money, you would chose the cheaper(but the same) product. 
For instance if the price of Orange Juice A went up by $2, but the price of Orange Juice B stayed the same, people would stop buying Orange Juice A, and the demand for OJ B would increase, even though its unrelated to the price change.

Multiplying frequency by period