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.