Question

Fifty thousand rupees is/are a big sum?

Answer 1

50,000 rupees is a big sum

Answer 2

Both- are as well as is- suggest the state of being as an auxiliary verb. They are one of the multiple forms of the verb "be," in accordance with the tense. Anyhow, here the statement would utilize "is" because fifty-thousand refers to a collective sum of fund which happens to be a single value, or say a single object. While the use of "are" requires a plural object to be placed in a sentence.

Now, the statement that would be generated after the analysis would be following:

Fifty-thousand rupees is a big sum.