A number can be added and subtracted by whole hundred, and numbers close to whole hundred can be regarded as whole hundred and whole ten. Finally, it is easier to add a few more and subtract a few more.

273-97 Take 97 as 100, first subtract 100, then add 3.

=273-( 100-3)

=273- 100+3

= 173+3

= 176

45 1-203 Divide 203 by 200+3.

=45 1-(200+3)

=45 1-200-3

=25 1-3

=248

419-271+81additive associative law

=4 19+8 1-27 1

=500-27 1

=229