document.write( "Question 1103607: A man can do a certain task in 21 hours, another man can do the task in 28 hours and a boy can do the task in 48 hours. Find how long it will take to do the task if all three work. \n" ); document.write( "
Algebra.Com's Answer #718292 by Edwin McCravy(20060)\"\" \"About 
You can put this solution on YOUR website!
>>>A man can do a certain task in 21 hours,
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document.write( "So the man's rate in jobs per hour is\r\n" );
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document.write( "1 job per 21 hours or\r\n" );
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document.write( "\"matrix%281%2C2%2C1%2Cjob%29%2Fmatrix%281%2C2%2C21%2Chours%29\",\r\n" );
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document.write( "so his rate in jobs/hour is \r\n" );
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document.write( "\"matrix%281%2C2%2C1%2F21%2Cjobs%2Fhour%29\"\r\n" );
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\n" ); document.write( ">>>another man can do the task in 28 hours,
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document.write( "So the other man's rate in jobs per hour is\r\n" );
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document.write( "1 job per 28 hours or\r\n" );
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document.write( "\"matrix%281%2C2%2C1%2Cjob%29%2Fmatrix%281%2C2%2C28%2Chours%29\",\r\n" );
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document.write( "so his rate in jobs/hour is \r\n" );
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document.write( "\"matrix%281%2C2%2C1%2F28%2Cjobs%2Fhour%29\"\r\n" );
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\n" ); document.write( ">>>a boy can do the task in 48 hours
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document.write( "That's 1 job per 48 hours or\r\n" );
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document.write( "\"matrix%281%2C2%2C1%2Cjob%29%2Fmatrix%281%2C2%2C48%2Chours%29\",\r\n" );
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document.write( "so his rate in jobs/hour is \r\n" );
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document.write( "\"matrix%281%2C2%2C1%2F48%2Cjobs%2Fhour%29\"\r\n" );
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\n" ); document.write( ">>>Find how long it will take to do the task if all three work.
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document.write( "Let x = the number of hours it will take all three to do the task.\r\n" );
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document.write( "Then their combined rate is\r\n" );
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document.write( "1 job per x hours or\r\n" );
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document.write( "\"matrix%281%2C2%2C1%2Cjob%29%2Fmatrix%281%2C2%2Cx%2Chours%29\",\r\n" );
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document.write( "so their combined rate in jobs/hour is \r\n" );
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document.write( "\"matrix%281%2C2%2C1%2Fx%2Cjobs%2Fhour%29\"\r\n" );
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document.write( "Their combined rate is also equal to the sum of their rates, so\r\n" );
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document.write( "Their combined rate = \"1%2F21%2B1%2F28%2B1%2F48\"\"%22%22=%22%22\"\"5%2F48\"\r\n" );
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document.write( "So we set them equal:\r\n" );
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document.write( "\"matrix%281%2C3%2C1%2Fx%2C%22%22=%22%22%2C5%2F48%29\"\r\n" );
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document.write( "Cross-multiply\r\n" );
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document.write( "\"matrix%281%2C3%2C5x%2C%22%22=%22%22%2C48%29\"\r\n" );
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document.write( "\"matrix%281%2C3%2Cx%2C%22%22=%22%22%2C48%2F5%29\"\r\n" );
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document.write( "\"matrix%281%2C3%2Cx%2C%22%22=%22%22%2C9%263%2F48%29\" hours\r\n" );
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document.write( "Edwin
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