1) If digits can not repeat, the number of possible variations is 10x9x8x7x6 =30,240
And the probaility of guessing is 1 / 30240 =0.000033
2) If digits can repeat an zero is no one of the digits, the number of possible variations is 9x9x9x9x9 = 9^5 = 59,049.
The probaility of guessing is 1/59,049 = 0.000017