Here, in units place,

6 + A = 3

Ie. A = - 3, which is not possible.

Hence A is greater than 10,where, 1 carry is given to tens place.

∴ A + 6 = 13

∴A = 7

Now, in tens place,

B + 9 + 1 = 7 … as 1 is carried

∴ B = -3, which is not possible.

Hence B is greater than 10,where, 1 carry is given to hundreds place.

∴ B + 9 + 1 = 17

∴ B = 7

Now, in hundreds place,

C + 6 + 1 = 1 … as 1 is carried

Ie. C = -6, which is not possible.

Hence C is greater than 10, where, 1 carry is given to thousands place.

∴ C + 6 + 1 = 11

∴ C = 4