document.write( "Question 829836: When Mr. Tan was his son's present age, his son was only 4 years old. When his son reaches his present age, Mr. Tan will be 79 years old. How old is each of them now? \n" ); document.write( "
Algebra.Com's Answer #500531 by Edwin McCravy(20056)\"\" \"About 
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document.write( "When Mr. Tan was his son's present age, his son was only 4 years old.  When his\r\n" );
document.write( "son reaches his present age, Mr. Tan will be 79 years old. How old is each of\r\n" );
document.write( "them now?\r\n" );
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document.write( "Let Mr. Tan's age be T.  \r\n" );
document.write( "Let his son's age be S.\r\n" );
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document.write( "When Mr. Tan was his son's present age, his son was only 4 years old.\r\n" );
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document.write( "Let y = the number of years ago it was when Mr. Tan was his son's \r\n" );
document.write( "present age. So we have two equations:\r\n" );
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document.write( "T - y = S, and S - y = 4\r\n" );
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document.write( "When his son reaches his present age, Mr. Tan will be 79 years old. \r\n" );
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document.write( "Let z = the number of years in the future it will be when his son\r\n" );
document.write( "reaches Mr. Tan's age.  So we have two more equations:\r\n" );
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document.write( "S + z = T, and T + z = 79.\r\n" );
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document.write( "We have four equations in four unknowns.\r\n" );
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document.write( "(1)  T - y = S\r\n" );
document.write( "(2)  S - y = 4\r\n" );
document.write( "(3)  S + z = T\r\n" );
document.write( "(4)  T + z = 79\r\n" );
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document.write( "Eliminate y from (1) and (2), and eliminate z from (3) and (4).\r\n" );
document.write( "Then solve the resulting two equations for T and S.\r\n" );
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document.write( "Answer:\r\n" );
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document.write( "S = 29,   T = 54,   y = 25,   z = 25.\r\n" );
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document.write( "Perhaps you could have seen right away that y and z are the same.\r\n" );
document.write( "Then you could have used only 3 unknowns.  And perhaps you could\r\n" );
document.write( "have seen that y and z were just the difference in their ages. If \r\n" );
document.write( "so, then you could have used only 1 equation and 1 unknown.  But \r\n" );
document.write( "it's easier to set up by using 4 equations and 4 unknowns.\r\n" );
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document.write( "Edwin
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