Multiple choice general knowledge math & puzzles

A certain street contains 100 buildings. They are numbered from 1 to 100. How many 9's are used in these numbers?

  1. 10

  2. 11

  3. 19

  4. 20
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Count all occurrences of the digit 9 in numbers 1-100. In the units place: 9, 19, 29, 39, 49, 59, 69, 79, 89, 99 (10 times). In the tens place: 90, 91, 92, 93, 94, 95, 96, 97, 98, 99 (10 times). The number 99 has two 9s and is counted correctly in both positions. Total: 10 + 10 = 20.

AI explanation

To find the number of times the digit 9 is used in the numbers from 1 to 100, we can analyze each position in the numbers.

  1. Units digit: There are 10 numbers that have a 9 in the units place - 9, 19, 29, ..., 89, 99.

  2. Tens digit: There are 10 numbers that have a 9 in the tens place - 90, 91, 92, ..., 98, 99.

So, in total, there are 10 + 10 = 20 numbers that contain the digit 9.

Therefore, the correct answer is option D) 20.

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