For my anniversary, I decided to surprise my wife. Since she is a voracious reader, I decided to collect a lot of books for her. On the first day of the month, I bought one book, on the second, I bought two and on the third, I bought three. This process went on till the anniversary and on the day, I had 231 books with me to gift her.
Can you calculate, on which day is our anniversary?  Options:
A) 31 B) 28 C) 23 D) 21 E) 25
D   21rd day   1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17 + 18 + 19 + 20 + 21= 
<gadget id="calculator">1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17 + 18 + 19 + 20 + 21</gadget>
<output>231</output>
231   Thus our anniversary falls on the 21rd day of the month.   You can apply other formulas to shorten the process but that is the simplest way to do it.
<result>D</result>
For my anniversary, I decided to surprise my wife. Since she is a voracious reader, I decided to collect a lot of books for her. On the first day of the month, I bought one book, on the second, I bought two and on the third, I bought three. This process went on till the anniversary and on the day, I had 231 books with me to gift her.
Can you calculate, on which day is our anniversary?