Since 1 Australian Dollar is equal to 0.684519857237 US Dollars, we could say that n Australian Dollars are equal to 0.684519857237 times n US Dollars. In other words, we could use the following formula:
US Dollars = Australian Dollars x 0.684519857237
For example, let's say that we want to convert 2 Australian Dollars to US Dollars. Then, we just replace Australian Dollars in the abovementioned formula with 2:
US Dollars = 2 x 0.684519857237
That is, 2 Australian Dollars are equal to 1.36903971447 US Dollars.