document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=1105385>Question 1105385</A>: <I><SPAN STYLE=\"word-break: break-all;\"> certain class contains 60% of boys and 40% of girls, all aged between 17 and 27 years. It is known that 80% of young people of both sexes are under 21 years of age and that among girls, 10% are over 21 years.\r<BR>\n" );
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document.write( "a) a person is chosen and it is verified that he is under 21 years. What is the probability of being a girl?<BR>\n" );
document.write( "b) a person is chosen at random. What is the probability of being a boy under the age of 21?<BR>\n" );
document.write( "c) choose a boy at random. How likely is he to be less than 21 years old? </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #720225 by </B><A HREF=/tutors/aboutme.mpl?userid=natolino_2017><B>natolino_2017(77)</B></A><img src=\"/images/stars/stars-08.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=natolino_2017><IMG SRC=http://www.algebra.com/images/aboutme-small.gif BORDER=0 alt=\"About Me\" VALIGN=MIDDLE></A>&nbsp;<IMG SRC=http://www.algebra.com/images/nopicture.jpg><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=720225\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> Let be the events A: The picked person is a Girl.<BR>\n" );
document.write( "                  B: the picked person is under 21 or 21.\r<BR>\n" );
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document.write( "First P(A) = 40%. P(B) = 80%.\r<BR>\n" );
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document.write( "a) P(A/B) = P(A&B)/P(B)=(90%(40%))/(80%)=9/20.<BR>\n" );
document.write( "b) P(A^c&B) = 60%(80%) = 12/25.<BR>\n" );
document.write( "c) P(B/A^c) = P(B&A^c)/P(A^c) = (12/25)/60% =4/5.\r<BR>\n" );
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