1/12th of the respondents responded with no opinion.
Since 2/3 agree and 1/4 disagreed, we need to subtract both of those numbers from one whole and see what was left over.
1 - 2/3 - 1/4
However, in order to solve, we need to give them common denominators.
12/12 - 8/12 - 3/12 = 1/12
