One estimate is 10 because
- 396 is close to 400
- 42 is close to 40
So 396/42 is close to 400/40 = 10
Using a calculator or long division shows that 396/42 = 9.43 approximately which isn't too far from 10.
----------------------------------------------
Another estimate is 8 because
- 396 is close to 400
- 42 rounds up to 50, when rounding up to the nearest tens
So 396/42 is somewhat close to 400/50 = 8
In some cases, dividing by 50 is easier than 40, but I think it's better to stick to the first estimate because it's closer but also because 400 and 40 can easily be divided mentally.