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 $50=n(A)+20-10\\n(A)=40.$
You can help us out by revising, improving and updating this answer.Update this answer
After 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.