You can do like that:
* After one month: 12000 + 0.1x12000 = 12000 + 1200 = 13200
* After two months: 13200 + 0.1x13200 = 13200 + 1320 = 14520
But the problem is when the number of months get big!
Then you can use $M = C(1 + i)^t$, $M$ is the final capital, $C$ is the inicial capital, $i$ is the rate and $t$ is the time (like how many months.)