Question 962934
1.{{{x+y+z=35}}}
.
.
.
{{{P=2C}}}
{{{12y=2(9x)}}
{{{12y=18x}}}

2.{{{2y=3x}}}
.
.
.
{{{9x+12y+15z=420}}}
3.{{{3x+4y+5x=140}}}
Multiplying eq. 1 by 5 and subtracting from eq. 3,
{{{3x+4y+5z-5x-5y-5z=140-175}}}
{{{-2x-y=-35}}}
Multiply by 3.
{{{-6x-3y=-105}}}
Substitute from eq. 2,
{{{-2(2y)-3y=-105}}}
{{{-4y-3y=-105}}}
{{{-7y=-105}}}
{{{y=15}}}
Then,
{{{2(15)=3x}}}
{{{x=10}}}
And,
{{{10+15+z=35}}}
{{{z=10}}}