Sorry for not posting any solution because this is just a wild guess. (By using trial and error.)
My answer is 5. If the original number is 5, and you place 1 before it, it will become 15, right? Then, it is said that the new number, which is 15, is 3 times as large as the original number. 3 times the original number, 5 is equal to 15.
So, if we consider it, this is technically, correct. ^^