Question

Housing Board built 100 flats. A painter is engaged in serially numbering each flat individually from 1 to 100. How many times will he be required to write ONE?

• A. 21
• B. 6
• C. 20
• D. 18

Answer 1

The correct option is A 21

Form $1$ to $100$ he would write ${}^{\prime }{1}^{\prime }$ in $20$ times
$1,10,11,12,13,14,15,16,17,18,19,21,31,41,51,61,71,81,91$ and $100$.

However, $11$ has two 1s.

Hence, the number increases to $21$.