\(\frac{1}{3}.x+\frac{3}{4}.x-75\%=-\frac{21}{4}\)

      \(\frac{1}{3}.x+\frac{3}{4}.x-\frac{3}{4}=-\frac{21}{4}\)

               \(x.\left(\frac{1}{3}+\frac{3}{4}\right)=-\frac{21}{4}+\frac{3}{4}\)

                              \(x.\frac{13}{12}=-\frac{9}{2}\)

                                       \(x=-\frac{9}{2}:\frac{13}{12}\)

                                        \(x=-\frac{54}{13}\)

=> x.(1/3+3/4) -75/100=-21/4

=> x .13/12   -  3/4     =21/4

=> x. 13/12                =21/4 + 3/4

=> x.13/12                 =6

=> x                          = 6 : 13/12

=> x                          = 72/13

vậy x= 72/13

       1/3 x X + 3/4 x X - 75% = -21/4

<=> 13/12 x X = -21/4 + 75/100

<=> 13/12 x X = -525/100 + 75/100

<=> 13/12 x X = -9/2

<=> X = -54/13

Bài 1 :

(4/5 + 3/10 - 1/5) x 6 : 4/7

= 9/10 x 6 : 4/7

= 27/5 : 4/7

= 189/20

2007 x 76 - 75 x 2007 - 2007

= 2007 x (76 - 75 - 1)

= 2007 x 0

= 0

(47,5 + 8,6 x 4,5 - 105)

= 47,5 + 38,7 - 105

= 86,2 - 105

= -94/5

Bài 2 :

a) x : 2/3 = 10

           x = 10 x 2/3

           x = 20/3

theo cách đó tự làm

a) 75% x X+ 3/4 = 30

              75%x X= 30-3/4

              75%x X=117/4

                        X=117/4 : 75/100

                        X=117/4 x 100/75

                        X= 39

\(75\%x+\frac{3}{4}x+x=30\)

\(\Leftrightarrow\left(75\%+\frac{3}{4}+1\right)x=30\)

\(\Leftrightarrow\frac{5}{2}x=30\)

\(\Leftrightarrow x=30:\frac{5}{2}\)

\(\Leftrightarrow x=12\)

đổi 75%=3/4.Ta có:

3/4xX+3/4xX+X=30

(3/4+3/4+1)xX=30

    5/2     x    X=30

                 X  = 30:5/2

                X   = 12

\(75\%\times x+\frac{3}{4}\times x+x=30\)

\(\frac{3}{4}\times x+\frac{3}{4}\times x+1\times x=30\)

\(\left(\frac{3}{4}+\frac{3}{4}+1\right)\times x=30\)

\(\frac{5}{2}\times x=30\)

\(x=30\div\frac{5}{2}\)

\(x=12\)

giá xăng từ 20 000 đồng lên 21 000 đồng 1 lít . hỏi giá xăng tăng bao nhiêu phần trăm? 

0,75 x X + 0,75 x X + X = 30

X x ( 0,75 + 0,75 + 1 ) = 30

X x 2,5 = 30

X = 30 : 2,5

X = 12

75%xX+3/4xX+X=30

<=>3/4xX+3/4xX+X=30

<=>10/4xX=30

<=>10xX=120

<=>X=120/10=12

Vậy X=12

0,75x + 0,75x = 30

0,75 . 2x = 30

2x = 40

x = 20

\(75\%\cdot x+\frac{3}{4}\cdot x+x=30\)

\(x\cdot\left(\frac{3}{4}+\frac{3}{4}+1\right)=30\)

\(x\cdot\frac{5}{2}=30\)

\(x=30\cdot\frac{2}{5}\)

\(x=12\)

\(75\%\cdot x+\frac{3}{4}\cdot x+x=30\)

\(\frac{3}{4}\cdot x+\frac{3}{4}\cdot x+x=30\)

\(\left(\frac{3}{4}+\frac{3}{4}\right)\cdot x+x=30\)

\(\frac{3}{2}\cdot x+x=30\)

\(\frac{3}{2}\cdot x+\frac{1}{1}\cdot x=30\)

\(\left(\frac{3}{2}+\frac{1}{1}\right)\cdot x=30\)

\(\frac{5}{2}\cdot x=30\)

\(x=30\text{ : }\frac{5}{2}\)

\(x=12\)