document.write( "<A HREF=/cgi-bin/jump-to-question.mpl?question=1083253>Question 1083253</A>: <I><SPAN STYLE=\"word-break: break-all;\"> A donut baker wants to ensure that the radius of its donuts is within +- 0.25cm.  Based on past statistical studies, she knows that the donut’s radius has a standard deviation of 1.25cm.  How many measurements should she take to ensure a 90% confidence in the radius of the donuts are within 0.25cm, rather than measure each and every donut that is baked?  </SPAN>\n" );
document.write( "</I><HR><TABLE width=100%><TR><TD><B><A HREF=http://www.algebra.com/>Algebra.Com's</A> Answer #697627 by </B><A HREF=/tutors/aboutme.mpl?userid=Boreal><B>Boreal(15235)</B></A><img src=\"/images/stars/stars-13.gif\" alt=\"\" >&nbsp;<A HREF=/tutors/aboutme.mpl?userid=Boreal><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/cgi-bin/show-face.mpl?user=Boreal><BR>You can <A HREF=\"/cgi-bin/embed-solution.mpl?show=1&solution=697627\">put this solution on YOUR website!</A><BR><SPAN STYLE=\"word-break: break-all;\"> The interval is +/-0.25 cm<BR>\n" );
document.write( "90%CI start with z and see if large enough not to use t<BR>\n" );
document.write( "1.645*1.25/sqrt(n)<=0.25<BR>\n" );
document.write( "multiply by sqrt(n)<BR>\n" );
document.write( "2.05625 <=0.25n<BR>\n" );
document.write( "square both sides<BR>\n" );
document.write( "4.23<0.0625 n<BR>\n" );
document.write( "divide by 0.0625<BR>\n" );
document.write( "67.65 which rounds up to 68.<BR>\n" );
document.write( "t for df=67, n=68, is 1.67<BR>\n" );
document.write( "1.67*1.25/sqrt(68)=0.253, which is just a little high.<BR>\n" );
document.write( "1.668*1.25/sqrt(69)=0.251<BR>\n" );
document.write( "use 70<BR>\n" );
document.write( "1.667*1.25/sqrt(70)=0.2491<BR>\n" );
document.write( "n=70 </SPAN>\n" );
document.write( "</TD></TR></TABLE>\n" );