Let's eliminate unknowns backwards, starting with
From the equations for
$G=C$
$F=B$
$E=1/2 B$
$D=1/2 A$
Substituting these in the remaining equations we get:
$S: A+C=3(1/2 A)+(1/2 B)+B = 3/2A + 3/2B$
$O: 4A+4B+4C = 12 (1/2 A)+4 (1/2 B)+4B+C = 6A + 6B+C$
Subtracting
$C=1/2A+3/2B$
Substitute this in our equation for
$4A+4B+4(1/2A+3/2B) = 6A+6B+(1/2A+3/2B)$
That is:
$6A+10B = 13/2A+15/2B$
Multiplying both sides by
$12A+20B = 13A+15B$
Subtract
$5B=A$
So if we put
$A=5B=10$
$B=2$
$C=1/2A+3/2B=5+3=8$
$D=1/2A = 5$
$E=1/2B = 1$
$F=B=2$
$G=C=8$
So:
$10FeSO_4+2KMnO+8H_2SO_4 -> 5Fe_2(SO_4)_3+K_2SO_4+2MnSO_4+8H_2O$