20 USD to AUD — 20 US dollars in Australian dollars

One US dollar is 1.43 Australian dollars.

20.00 US dollars is 28.54 Australian dollars. The working is 20.00 × 1.427221 = 28.54.

Because this is the mid-market reference rate — the midpoint between what buyers and sellers were quoting. Banks, card networks and bureaux add a margin to it, typically between 0.5% and 4%. Treat the figure here as the benchmark you compare their offer against, not the amount you will receive.

Once per working day. The European Central Bank publishes its reference rates at around 16:00 Central European Time, and there is no update at weekends or on ECB holidays — those days show the last published figure.

For euro-area reporting, generally yes: ECB reference rates are the standard published source and many tax authorities accept them. Check the rule that applies to you, since some require a specific date's rate or a national central bank's figure instead.

The ECB publishes around thirty currencies, chosen for their relevance to euro-area trade. Currencies outside that set are not covered here rather than being estimated from a less reliable source.