document.write( "Question 517531: Mary is 3 years older than her brother Tom, and Tom is 4 years older than John. Three years ago Mary was twice as old as John. How old is each now? \n" ); document.write( "
Algebra.Com's Answer #344834 by Maths68(1474)\"\" \"About 
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Let
\n" ); document.write( "Present age of Tom = t
\n" ); document.write( "Present age of John = j
\n" ); document.write( "Present age of Mary =m\r
\n" ); document.write( "\n" ); document.write( "Mary is 3 years older than her brother Tom
\n" ); document.write( "m=t+3............(1)
\n" ); document.write( "Tom is 4 years older than John
\n" ); document.write( "t=j+4.............(2)
\n" ); document.write( "Three years ago Mary's age was (m-3) and John was (j-3)
\n" ); document.write( "Then Mary was twice as old as John
\n" ); document.write( "m-3=2(j-3)
\n" ); document.write( "m-3=2j-6
\n" ); document.write( "m=2j-6+3
\n" ); document.write( "m=2j-3.............(3)\r
\n" ); document.write( "\n" ); document.write( "We have three equations and three unknowns (m,t,j) and we can solve these equation either by substitution method or elimination method
\n" ); document.write( "m=t+3............(1)
\n" ); document.write( "t=j+4.............(2)
\n" ); document.write( "m=2j-3.............(3)\r
\n" ); document.write( "\n" ); document.write( "Put the value of m from (1) to (3)
\n" ); document.write( "t+3=2j-3
\n" ); document.write( "t=2j-3-3
\n" ); document.write( "t=2j-6...............(4)
\n" ); document.write( "Put the value of t from (2) to (4)
\n" ); document.write( "j+4=2j-6
\n" ); document.write( "j-2j=-6-4
\n" ); document.write( "-j=-10
\n" ); document.write( "Multiply by -1 both sides of above equation
\n" ); document.write( "j=10
\n" ); document.write( "Put the value of j in (2)
\n" ); document.write( "t=j+4
\n" ); document.write( "t=10+4
\n" ); document.write( "t=14
\n" ); document.write( "Put the value of t in (1)
\n" ); document.write( "m=t+3............(1)
\n" ); document.write( "m=14+3
\n" ); document.write( "m=17\r
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\n" ); document.write( "\n" ); document.write( "Present age of Tom = t = 14 years old
\n" ); document.write( "Present age of John = j = 10 years old
\n" ); document.write( "Present age of Mary =m = 17 years old\r
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