im stuck - its for some extra credits

x+y+z+a = 16480

x=y+ 1\2z

y=a-z

z=3x

a=y+z

find a,x,y,z

See i try subbing into the top but i always have too many variable and anything else at the point undos something. I think i am missing a place where i can simplify. Please help. Any tips or anything will be appreciated.

extra credit problem i got :(

x+y+z+a = 16480

x=y+ 1\2z

y=a-z

z=3x

a=y+z

find a,x,y,z

See i try subbing into the top but i always have too many variable and anything else at the point undos something. I think i am missing a place where i can simplify. Please help. Any tips or anything will be appreciated.