Đặt 

• 5,4:0,4 × 1420 + 4,5 × 780 × 3

= 13,5 × 1420 + 13,5 × 780
= 13,5 × 1420 + 780
= 13,5 × 2200
= 29700

• 3 + 6 + 9 + 12 + 15 + 18 + 21 + 24 + 27

= 3 + 27 + 6 + 24 + 9 + 21 + 12 + 18 + 15
= 30 + 30 + 30 + 30 + 15
= 30 × 4 + 15
= 120 + 15
= 135

\(\Rightarrow\)\(\frac{29700}{135}\)

\(\frac{\text{5,4:0,4x1420+4,5x780x3}}{3+6+9+12+15+18+21+24+27}\)=220

\(5,4:0,4\times1420+4,5\times780\times3\)

\(=13,5\times1420+13,5\times780\)

\(=13,5\times\left(1420+780\right)\)

\(=13,5\times2200\)

\(=29700\)

\(3+6+9+12+15+18+21+24+27\)

\(=\left(3+27\right)+\left(6+24\right)+\left(9+21\right)+\left(12+18\right)+15\)

\(=30+30+30+30+15\)

\(=30\times4+15\)

\(=120+15\)

\(=135\)

cau 1 sai de

cau 2 :

3+6+9+12+15+18+21+24+27= (3+27)+(6+34)+(9+21)+(12+18)+15= 30+30+30+30+15=135

A=1498

\(\frac{5,4:0,4\times1420+4,5\times780\times3}{3+6+9+12+15+18+21+24+27}\)

\(=\frac{13,5\times1420+13,5\times780}{\left(3+27\right)+\left(6+24\right)+\left(9+21\right)+\left(12+18\right)+15}\)

\(=\frac{13,5\times\left(1420+780\right)}{30+30+30+30+15}\)

\(=\frac{13,5\times2200}{135}\)

\(=\frac{29700}{135}\)

\(=220\)

A = 135 × 1420 + 4 , 5 × 780 × 3 3 + 6 + 9 + . . . + 24 + 27 = 135 × 1420 + 13 , 5 × 780 3 + 7 + 6 + 24 + 9 + 21 + 21 + 18 + 15 = 135 × 142 + 78 30 × 4 + 15 = 135 × 220 135 = 220

Vậy A = 220

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