I have been asked to write a subroutine that allows the user to query exchange rates across
three currencies.

I have prepared a spreadsheet that stores a table of exchange
rates across three currencies such that the user can fill in before the program
runs.

At run time the user simply states the source and destination currencies
and the sub should output to the screen the relevant exchange rate
picked out from the table.

Any ideas or pointers on how I can achieve this?? Thx.