Answer
$17$
Work Step by Step
According to the counting formula we know that $n(A\cup B)=n(A)+n(B)-n(A\cap B).$
Hence here $n(A\cup B)=n(A)+n(B)-n(A\cap B)=8+12-3=17.$
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.