a) (125 - x) + 25 = 98

125 - x = 98 - 25

125 - x = 73

x = 125 - 73

x = 52

b) 100 : (x + 20) = 68 : 34

100 : (x + 20) = 2

x + 20 = 100 : 2

x + 20 = 50

x = 50 - 20

x = 30

c) (3x + 12) . 5 = 25 . 4 - 10

(3x + 12) . 5 = 100 - 10

(3x + 12) . 5 = 90

3x + 12 = 90 : 5

3x + 12 = 18

3x = 18 - 12

3x = 6

x = 6 : 2

x = 3

d) 210 : (2x - 3) - 20 = 10

210 : (2x - 3) = 10 + 20

210 : (2x - 3) = 30

2x - 3 = 210 : 30

2x - 3 = 70

2x = 70 + 3

2x = 73

x = 73/2

a, ( 125 - x ) + 25 = 98

⇒ 125 - x = 73

⇒ x = 52.

Vậy..

b, 100 : ( x + 20 ) = 68 : 34

⇒ 100 : ( x + 20 ) = 2

⇒ x + 20 = 50

⇒ x = 30

Vậy...

c, ( 3 . x + 12 ) . 5 = 25 . 4 - 10

⇒ ( 3 . x + 12 ) . 5 = 90

⇒ 3 .x + 12 = 18

⇒ 3x = 6

⇒ x = 2.

Vậy...

d, 210 : ( 2 .x - 3 ) - 20 = 10

⇒ ( 2 .x - 3 ) - 20 = 21

⇒ 2x - 3 = 41

⇒ 2x = 44

⇒ x = 22.

Vậy..

\(\Leftrightarrow20+8\left(x+3\right)=100\\ \Leftrightarrow8\left(x+3\right)=80\\ \Leftrightarrow x+3=10\\ \Leftrightarrow x=7\)

a) \(18+25\times4-4^3\)

\(=18+100-64\)

\(=118-64\)

\(=54\)

b) \(275-\left(49+125\div5^3\right)\)

\(=275-\left(49+125\div125\right)\)

\(=275-\left(49+1\right)\)

\(=275-49-1\)

\(=226-1\)

\(=225\)

c) \(2015+\left(8\times15-\left(18-8\right)^2\right)\)

\(=2015+\left(8\times15-\left(10\right)^2\right)\)

\(=2015+\left(8\times15-100\right)\)

\(=2015+\left(120-100\right)\)

\(=2015+20\)

\(=2035\)

a, 165:11.999 + 85.999 = (15.11):11.999 + 85.999 = [(15.11):11].999 + 85.999

= 15.(11:11).999 + 85.999 = 15.999 + 85.999 = (15+85).999 = 99900

b, 25.4 + 25:5 – 4.(30 – 5) – 5 = 25.4 + 25:5 – 4.25 – 5 = (25.4 – 4.25) + (25:5 – 5) = 0 + 0 = 0

`\(=\frac{5.5.4^{15}.9^9-2^9.6^{18}.6^2}{5.3.4^{12}.2.9^9.3-2^9.2^2.6^{18}.11}\)

\(=\frac{5.4^3-6^2}{3.2.3-2^2.11}\)

\(=\frac{320-36}{18-44}\)

\(=\frac{-284}{26}=\frac{-142}{13}\)

Gấp lắm ạ

Bài 2: 

a: \(\left(6x-39\right):7=3\)

\(\Leftrightarrow6x-39=21\)

hay x=10

a) x*15/2/3=5

x=5*3*2*15

x=450 

đúng cho mình nha 

x . 15 : 2 : 3 = 5

x . 15 : 2 = 5 . 3

x . 15 : 2 = 15

x  . 15 = 15 x 2

x . 15 = 30

=> x = 2.

25 . 4 + (150.x) = 200

100 + (150.x) = 200

150 . x = 200 - 100

150 . x = 100

=> x = 0,6666...