1.Tính nhanh a. 91.25 -91 . 13 - 91.12 b.236 : 3 + 64 : 3 c. 47 . 8 - 27 .9 + 47 . 12 - 27 -11 d. 375 : 25 - 125 : 25 2. tìm x a,(x-25)-175=0 b,485-(6x=60)=5 c,315+(135-x)=450 d,346+(210-x)=556

Bài `1`

`a ) 91 . 25 - 91 . 13 - 91 . 12`

`= 91 . ( 25 - 13 - 12 )`

`= 91 . 0`

`= 0`

`b ) 236 : 3 + 64 : 3`

`= ( 236 + 64 ) : 3`

`= 300 : 3`

`= 100`

`c ) 47 . 8 - 27 . 9 + 47 . 12 - 27 . 11`

`= 47 . ( 8 + 12 ) - 27 . ( 9 + 11 )`

`= 47 . 20 - 27 . 20`

`= ( 47 - 27 ) . 20`

`= 20 . 20`

`= 400`

`d ) 375 : 25 - 125 : 25`

`= ( 375 - 125 ) : 25`

`= 250 : 25`

`= 10`

Bài `2`

`a ) ( x - 25 ) - 175 = 0`

`x - 25 = 0 + 175`

`x - 25 = 175`

        `x = 175 + 25`

        `x = 200`

`b ) 485 - ( 6 . x + 60 ) = 5`

`6 . x + 60 = 485 - 5`

`6 . x + 60 = 480`

`6 . x = 480 - 60`

`6 . x = 420`

     `x = 420 : 6`

     `x = 70`

`c ) 315 + ( 135 - x ) = 450`

`135 - x = 450 - 315`

`135 - x = 135`

          `x = 135 - 135`

          `x = 0`

`d ) 346 + ( 210 - x ) = 556`

`210 - x = 556 - 346`

`210 - x = 210`

          `x = 210 - 210`

          `x = 0`

`#dtkc`