Given a+b+c+d=45
and a+2=b-2=2c=d/2
compute a+b+c-d
a=d/2-2
b=d/2+2
c=d/4 --> a+b+c+d=2d+d/4=45-->d=20
a+b+c-d=d/4=5
and a+2=b-2=2c=d/2
compute a+b+c-d
b=d/2+2
c=d/4 --> a+b+c+d=2d+d/4=45-->d=20
a+b+c-d=d/4=5