 # How many integers in the set - Quantitative Aptitude – Equation

## CAT 2020 - Slot 3 - Quantitative Aptitude - Equation – Question 16 - How many integers in the set

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

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 9except a and b Number ofPossibilities 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

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