Use rounding or compatible numbers to estimate 222 plus 203

200 can be rounded down to 200, which is easy to add with. Thus, an estimate is 222+200=422.

Alternatively, you could round 222 up a bit, to get 225+200=425. This is actually the correct answer, but this rounding doesn't make the calculation easier, so if you needed a quick estimate I'd go with the first option.