LOC/USD and USD/LOC are currency pairs. Whatever currency comes first is known as the base currency. The currency that comes second is known as quote currency. For major currency pairs, the format of the currency pair is based on established priority. For example, EUR has the highest established priority and thus is almost always shown as the base currency.
How long can I – coming from country X – live in country Y by spending my last monthly salary converted by nominal exchange rate:
1 as a backpacker or as a convenience tourist?
2 adapting to the other country's standards which may be lower?
3 adapting to higher standards than I am used to?
4 How much does this period deviate from a month?
This is tricky to answer precisely because governments vary a lot in the services they extent to visitors. There is also variation in the mix of consumption goods across countries as well as their relative prices. You might respond to local conditions by changing your consumption bundle more or less than the average person and that would change the economics ...
I think the World Bank's table Price level ratio of PPP conversion factor (GDP) to market exchange rate gives an answer. The numbers given here indicate somehow how much of a month an average inhabitant of this country could afford to live an average life in the United States (by an average monthly income in his own country), right?
For the "poorest" ...
It works like any other exchange rate:
Divide ₹50,000 by 70 to get about US$714 / month at market exchange rates
Divide ₹50,000 by 18 to get about PPP$2778 / month at PPP rates (so not a poor farmer)
Multiply USD$30 by 70 to get ₹2100 at market exchange rates
Multiply PPP$30 by 18 to get ₹540 at PPP rates