21.8*2.9+3.99[简算]
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21.8*2.9+3.99

=21.8*(3-0.1)+(4-0.01)

=21.8*3-21.8*0.1+4-0.01

=65.4-2.18+4-0.01

=(65.4+4)-(2.18+0.01)

=69.4-2.19

=67.21

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