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Bài `6:` Tính

`a) 258 - 65 = 193`

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`b) 478 - 256 + 47 

`= 222 + 47`

`= 269`

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`c) 115 xx 13 - 13 xx 15`

`= 13 xx ( 115 - 15 )`

`= 13 xx 100`

`= 1300`

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`d) 567 + 421 - 147 - 45`

`= ( 567 - 147 ) + ( 421 - 45 )`

`= 420 + 367`

`= 787`

Bài `7:` Tìm `x`

`a) 12 + x = 56`

`x = 56 - 12`

`x = 44`

Vậy `x = 44`

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 `b) 25 - x = 14`

`x = 25 - 14`

`x = 11`

Vậy `x = 11`

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`c) x - 157 = 458`

`x = 458 + 157`

`x = 615`

Vậy `x = 615`

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`d) 255 - ( x + 9 ) = 184`

`x + 9 = 255 - 184`

`x + 9 = 71`

`x = 71 - 9`

`x = 62`

Vậy `x = 62`

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`e) 541 + ( 218 - x ) = 678`

`218 - x = 678 - 541`

`218 - x = 137`

`x = 218 - 137`

`x = 81`

Vậy `x = 81`

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`f) ( x - 36 ) - 133 = 14`

`x - 36 = 14 + 133`

`x - 36 = 147`

`x = 147 + 36`

`x = 183`

Vậy `x = 183`

Bài `8:` Tính hợp lý

`a) 14 xx 50`

`= 7 xx 2 xx 50`

`= 7 xx 100`

`= 700`

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`b) 16 xx 125`

`= 2 xx 8 xx 125`

`= 2 xx 1000`

`= 2000`

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`c) 9 xx 24 xx 25`

`= 9 xx 6 xx 4 xx 25`

`= 9 xx 6 xx 100`

`= 54 xx 100`

`= 5400`

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`d ) 30 xx 40 xx 50 xx 60`

`= 3 xx 10 xx 4 xx 10 xx 5 xx 10 xx 6 xx 10`

`= 1000 xx ( 3 xx 4 xx 5 xx 6 )`

`= 1000 xx 360`

`= 3600000`

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`e) 64 xx 125 xx 875`

`= 8 xx 8 xx 125 xx 125 xx 7`

`= 8 xx 125 xx 8 xx 125 xx 7`

`= 1000 xx 1000 xx 7`

`= 1000000 xx 7`

`= 7000000`