This question is from Permutation and Combination. From a set of integers, we need to find the numbers of integers that have at least one digit repeated. CAT Permutation and Combination and Probability is an important topic in the **CAT Exam**. A range of questions can be formed from this topic. In **CAT Exam**, one can generally expect to get 2~3 questions from these topics.

Question 16 : How many integers in the set {100, 101, 102, ..., 999} have at least one digit repeated?

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The total number of integers from 100 to 999 is 999 - 99 = 900.

Of these 900 integers, if we could find the number of integers which do not have repetitions in them, we can also find the number of integers that have repetitions.

Let's find the number of integers that between 100 and 999, which do not have repetitions.

a | b | c | |

Possibilities | 1 to 9 | 0 to 9 except a | 0 to 9 except a and b |

Number of Possibilities | 9 | 9 | 8 |

The number of integers that between 100 and 999, which do not have repetitions = 9 × 9 × 8

Therefore, the number of integers that have repetitions = 900 - 9 × 9 × 8

= 9 (100 - 9 × 8)

= 9 (28)

= 180 + 72 = 200 + 52 = 252

The question is **"How many integers in the set {100, 101, 102, ..., 999} have at least one digit repeated?" **

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