Question 1152557
Just follow the description literally and then solve.


The description as written means exactly:
{{{system(y+z=3x,z=y+15,x+y+z=180)}}}
Solve this system.


Not the only way to go but:

{{{system(3x-y-z=0,y-z=-15,x+y+z=180)}}}


Do E1+E2, and E3-E2, and have
{{{system(3x-2z=-15,x+2z=195)}}}

ADD:
{{{(3x-2z)+(x+2z)=-15+195}}}

{{{3x+x+0=180}}}

{{{4x=180}}}

{{{x=180/4=(2*5*2*9)/(2*2)}}}

{{{highlight(x=45)}}}, and you can find the other two.