11. (6 points) Natural numbers $1,2,3, \cdots$ are written down consecutively to form a number $123456789101112 \cdots$. When a certain number is reached, the formed number is exactly divisible by 72 for the first time. This number is $\qquad$ _. $\qquad$
【Answer】Solution: Since $72=8 \times 9, 8$ and 9 are coprime, any multiple-digit number formed by 9 consecutive natural numbers can definitely be divided by 9,

then when it is $9, 18, 27, 36, 45, \cdots$, it can be divided by 9.
Since $9, 18, 27, 36, 45, \cdots$ are themselves multiples of 9,
thus when it is $8, 17, 26, 35, 44, \cdots$, it can also be divided by 9.
Since $678, 718, 526$ cannot be divided by 8, but 536 can be divided by 8,
therefore, this natural number is 36.
Answer: This natural number is 36.