9's in the city
Logic
Level
2
A certain city contains 100 buildings. They are numbered from 1 to 100. How many 9's are used in all these numbers?
The answer is 20.
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1 solution
From 1 to 89 (both numbers included) there are nine 9s.
From 90 to 98 (both numbers included) there are 9 nines.
And 99 has 2 nines. 2 + 9 + 9 = 20