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5,6,6,9, (), 81 a.12 b.15 c.18 d.21 consultation process
5×6÷5=6
6×6÷4=9
6×9÷3=18
9×18÷2=81
So, choose c.18
Simple calculation: 79.8 × 80.2 =?
79.8×80.2
=(80-0.2)(80+0.2)
=80^2-0.2^2
=6400-0.04
=6399.96
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