Question 1076410
There is an excellent explanation here:
https://www.algebra.com/algebra/homework/Probability-and-statistics/Probability-and-statistics.faq.question.1052291.html
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(Hint: If you type your question into a search engine and add site:algebra.com, many standard questions have already been solved.  That way, you can use your two questions a day for additional problems! :-))