We can solve this problem using Fundamental Principle of Multiplication. Fundamental Principle of Multiplication: If there are two jobs such that one of them can be completed in m ways, and when it has been completed in any one of these m ways, second job can be completed in n ways; then two jobs in succession can be completed in m × n ways. By using this definition we can find the number of words having at least one letter is repeated. Given, the number of three digit number-locks. Total number of 3 digit numbers =10 × 10 × 10 = 1000 Total number of 3 digit numbers in which no digit repeated = 10 × 9 × 8 = 720 At least one of their digits repeated = 1000 - 720 = 280 Therefore, the number of three digit number-locks having atleast one of their digits repeated is 280.