125.

Balance the following equation by oxidation number method:

(i) The Skeleton equation along with oxidation number of each atom is
                +4      -2            0            0            +2             -2
               Sn        O₂   +     C       Sn+         C              O

(ii) Noting the change in O.N. of above atoms,
    Increase           C = 0 to  +2   i.e. 2
   Decrease           Sn = +4 to 0  i.e. 4

(iii) Equalise the increase/decrease in O.N. by multiplying SnO₂ by 1 and C by 2
                   SnO₂ + 2C    Sn + CO

(iv) On counting and balancing the atoms on both sides, we get the balanced equation:
                     SnO₂ + 2C   Sn + 2CO