Question

if x+y+z=50 and x+5y+10z=100. Find x,y z

Answer 1

x+y+z=50
       x=50-y-z
x+5y+10z=100
 50-y-z+5y+10z=100
 4y+9z=50
 y=8
 z=2
 so,x=40   

Answer 2

(we will take x because in the next equation there no coefficient of x)

x+y+z=50
x= 50-y-z                       (transposing y and z )

x+5y+10z=100
(50-y-z) + 5y +10z = 100     (replacing x with (50-y-z) )
50 + (5y-y) + (10z-z) = 100
4y + 9z = 100-50
4y + 9z = 50

so 
 y=8
z=2
x=40