B2. Jure wrote down the natural numbers from 1 to 2015 on a board. Urška then reviewed the written numbers from the smallest to the largest and erased every number that was not divisible by 3. Among the numbers that remained on the board, she then erased every number that was not divisible by $3^{2}$ from the smallest to the largest. Among the remaining numbers, she then erased every number that was not divisible by $3^{3}$ from the smallest to the largest, and so on. Which number did Urška erase last?
B2. After Urška first reviewed all the recorded numbers, only numbers divisible by 3 remained on the board. When she went through the numbers a second time, only numbers divisible by $3^{2}$ remained. After the third review, only numbers divisible by $3^{3}$ remained, and so on. The largest power of 3 that is less than 2015 is $3^{6}=729$, since $3^{7}=2187$. Thus, after Urška's sixth review of the numbers, only numbers divisible by 729 remained on the board, which are 729 and 1458. In the next review, she erased both of these numbers, as they are not divisible by 2187. As the last step, Urška erased the number 1458.

## Discovery that after the first review, only numbers divisible by 3 remained

Discovery that after the third review, only numbers divisible by $\mathbf{s} 3^{3}$ remained

1 point
Justified conclusion that the largest power of 3 less than 2015 is
the number $3^{6}=729$ ..... 2 points
Discovery that after the sixth review, only the numbers
729 and 1458 remained ..... 1 point
Correct answer ..... 1 point

(If the contestant writes only the correct solution without arguments, only 1 point is awarded.)