Known: X 1+X2+X3=3X0.
Add up these three figures, and it is:
3X 1+5 + 3X2+5 + 3x3+5
=3(x 1+x2+x3)+ 15
=3*3x0+ 15
=3(3x0+5)
Average and divide by 3.
Equal to 3x0+5.