April 25th, 2014, 02:40 PM  #1 
Senior Member Joined: Nov 2013 Posts: 434 Thanks: 8  fin
3x+5y+z=40 7x+12y+2z=92 find x+y+z 
April 25th, 2014, 02:58 PM  #2 
Math Team Joined: Dec 2013 From: Colombia Posts: 7,671 Thanks: 2651 Math Focus: Mainly analysis and algebra 
Write $u = x + y + z$ and $v = x + 2y$ and solve for $u$ and $v$

April 25th, 2014, 03:27 PM  #3 
Global Moderator Joined: Dec 2006 Posts: 20,809 Thanks: 2150 
Subtracting twice the second equation from five times the first equation gives x + y + z = 200  184 = 16. 