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Exchange rates are often not perfectly aligned, even if for a short periods of time. This is where arbitrage comes into play and levels the market.

With that we can go back to the example:

"A" = 2.0000 "B"

"B" = 3.0000 "C" -> "A" = 6.0000 "C"

"A" = 7.0000 "C"

If you notice this happening in the real world, you have the opportunity to:

1. buy 7 "C" with 1 "A"

2. buy 2 "B" with 6 "C"

3. buy 1 "A" with 2 "B"

4. keep 1 "A" profit

But it's rarely that simple and it often involves 3 or more currencies