We find the number of SUCCESSFUL ways, those which end in 3. That will be the numerator of the probability: Choose the most restrictive thing first. That's the last or 4th digit 3. Choose the 4th digit 1 way, as 3. Choose the 1st digit any of the remaining 3 ways, as 1, 2 o 4. Choose the 2nd digit either of the 2 remaining ways. Choose the 3rd digit as the 1 remaining way. Number of SUCCESSFUL identification numbers = 3×3×2×1 = 18 ------------------------------ We find the number of POSSIBLE ways, those with no restrictions That will be the denominator of the probability: Choose the 1st digit any of the 4 ways. Choose the 2nd digit any of the 3 remaining ways. Choose the 3rd digit as either of the 2 remaining ways. Choose the 4th digit as the 1 remaining way. Number of POSSIBLE identification numbers = 4×3×2×1 = 24 ---------------------------- The probability is 18 SUCCESSFUL ways out of 23 POSSIBLE ways. "OUT OF" means "OVER" or "DIVIDED BY". Probability of a SUCCESS = 18/24 which reduces to 3/4. Edwin