Make the smallest 4 digit number by using any one digit twice from the following digits
5, 8, 1

The correct answer is 1158

To make the smallest number, let us first arrange the numbers in ascending order.
⇒ 1, 5, 8
Since it is required to make a 4 digit number, we need to take the smallest digit and use it twice i. e. 1 in this case.
⇒ 1, 1, 5, 8 and the number is 1158