Please simplify this problem

A. The missing number is 11

Step-by-step explanation:

The highest number in the set is 30 and the lowest is 12, this makes the range 18.

30 is supposted to be there to make the range what the problem says it is: 12.

20 and 25 could be there because no matter what, as long as 30 is the greatest number, 20 and 25 don't effect the range AT ALL.

If 11 was in the number sequence, the lowest number would be 11, making the range 19, (30-11=19) which is not what the teacher is saying the range is.

So in short, A. The missing number is 11 is the correct answer.