Ask for a perfect fifth power of a number from 1 to 50.
Then give the fifth root of that number!

Note that when you take a fifth power of any number,
the units digit of the number and the fifth power are the same.

So, if the person gives you a number less than 100,000,
    you answer is just the last digit.
If the person gives you a number greater than or equal to 100,000 and less than 3,000,000,
    you answer is a one and the last digit.
If the person gives you a number greater than 3,000,000 and less than 24,000,000,
    you answer is a two and the last digit.
If the person gives you a number greater than 24,000,000 and less than 100,000,000,
    you answer is a three and the last digit.
If the person gives you a number greater than 100,000,000 and less than 300,000,000,
    you answer is a four and the last digit.
If the person gives you a number greater than 3,000,000,
    you answer is 50.