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 the digit 9 appearing in every position from 1 to 100. The units place has 10 occurrences (9, 19, 29, 39, 49, 59, 69, 79, 89, 99). The tens place from 90-99 has 10 more occurrences (90, 91, 92, 93, 94, 95, 96, 97, 98, 99). Total: 10 + 10 = 20. The number 99 contains two 9s, so it counts twice.

AI explanation

To find out how many 9's are used in the numbers from 1 to 100, we need to consider each digit position separately.

  1. For the units digit: Out of the numbers from 1 to 100, 9 is used as the units digit in 9, 19, 29, ..., 99. So, there are 10 numbers where 9 is used as the units digit.

  2. For the tens digit: Out of the numbers from 1 to 100, 9 is used as the tens digit only in 90, 91, 92, ..., 99. So, there are 10 numbers where 9 is used as the tens digit.

Therefore, the total number of 9's used in the numbers from 1 to 100 is 10 + 10 = 20.

Hence, the correct answer is option D) 20.

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