read it aloud: fifty-eight. The 5 stands for "50" or fifty.
another way to look at it is to count the position at which 5 is: it's the second position from the right. In our counting system, the places have the value of 10 to the power of (number of the position minus 1): the position is 2: so the value is 5(the digit)*10^(2-1), which is 5*10^(1)=5*10, which is 50