Although the Lambda Calculus - Evaluating Expressions #3 is probably the most simple of all the functional problems on Hackerrank (it is quite easy to solve it and even more easy to guess the right answer), it references the precious gem of the functional world: Church encoding.

Under Church encoding, what natural number does the expression $$λx.λy.x^{47}y$$ represent?

Of course, the answer is 47. But the Church encoding is very nice, indeed.