Their answer is incorrect for front-end estimation (which in algebra isn't supposed to give you a 'right answer' but is used simply as a means for checking your calculations to ensure you haven't gone off the tracks somewhere). In front-end estimation, you do start with the first digit, but you always go to the second digit. A 22% variance isn't a 'reasonable estimation'. The textbook way to solve this would be .

354+291

300+200=500

50+90=140

500+140=640

That way when you look for the correct true answer of 645, you can compare it to the front-end estimation of 640 and know you are most likely correct.

It really is somewhat useless with small numbers like this but you get the point. This lesson's biggest flaw is it allows for such a large variance as being correct. If you were adding two numbers and saw a 22% difference in your reasonable estimate and the answer you came up with, you would assume one is wrong.