I saw this sentence in the dictionary on my phone(so I can't get a link to it here):
We're only getting 130 yen to the dollar at the moment.
I'm wondering why the definite article 'the' is being used instead of indefinite article 'a'? Can we instead say: We're only getting 130 yen to a dollar at the moment?
I've also seen other examples in which 'the' is used instead of 'a':
The car will do over 40 miles to the gallon.
The scale of your map is one inch to the mile.
However, I did see an example in which 'a' is used instead of 'the':
There are just over four and a half litres to a gallon.
Can someone help to explain why sometimes 'a' is used and sometimes 'the' is used? Or they are just interchangeable in those scenarios?