How many positive integers less than 700 have no repetition of digits?

Important: The integers in question can be 3-digit integers, 2-digit integers, and 1-digit integers. So we should consider three cases:

Case i: 3-digit integers that are less than 700 and have no repetition of digits)
The first digit can be 1, 2, 3, 4, 5, or 6. So, there are 6 different ways to choose the first digit.
The second digit can be any of the digits from 0 to 9, EXCEPT the digit chosen for the first digit. So, there are 9 different ways to choose the second digit.
The third digit can be any digit EXCEPT the 2 digits chosen as the first and second digits. So, there are 8 different ways to choose the third digit.
Total number of 3-digit integers = (6)(9)(8) = 432

Case ii: 2-digit integers that no repetition of digits)
The first digit can be any digit other than 0. So, there are 9 different ways to choose the first digit.
The second digit can be any of the digits from 0 to 9, EXCEPT the digit chosen for the first digit. So, there are 9 different ways to choose the second digit.
Total number of 2-digit integers = (9)(9) = 81

Case iii: 1-digit integers
This can be any digit from 1 to 9
Total number of 1-digit integers = 9

TOTAL = 432 + 81 + 9 = 522

Answer: B