Answer
$40.$
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 $50=n(A)+20-10\\n(A)=40.$
Precalculus (10th Edition)
by Sullivan, Michael
Published by
Pearson
ISBN 10:
0-32197-907-9
ISBN 13:
978-0-32197-907-0
Chapter 13 - Counting and Probability - 13.1 Counting - 13.1 Asses Your Understanding - Page 847: 13
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