Use the properties for radicals to simplify the espression. 
Assume all variables represent positive real numbers.

sqrt 5/3

Yes that IS a real number.  Only when there's a negative number
under the radical do you not have a real number.

Checkley71 above only gave you a decimal approximation,
but I'm sure your teacher must not have wanted that, but
the simplest radical form instead, which is found by
rationalizing the denominator:   

     ____
    / 5
\  / ---
 \/   3

To rationalize the denominator, choose the smallest
integer that you can multiply the denominator 3 by to get
a perfect square.  That number is 3, since 3·3 = 9, which is
a perfect square.  So we multiply under the radical by 3/3,
which will not change the value because 3/3 is just 1, and
you don't change the value when you multiply by 1.

     ________
    / 5   3
\  / ---×---
 \/   3   3

Multiply numerators and denominators:


     _____
    / 15
\  / ----
 \/    9

Now we take square roots of the numerator
and denominator:
    __
   Ö15
  -----
    Ö9
                         _
Replace the denominator Ö9 by 3

    __
   Ö15
  -----
    3

Edwin