b: Ta có: \(\dfrac{12}{5}:x+\dfrac{4}{3}=3+\dfrac{2}{3}\)

\(\Leftrightarrow\dfrac{12}{5}:x=3-\dfrac{2}{3}=\dfrac{7}{3}\)

hay \(x=\dfrac{12}{5}:\dfrac{7}{3}=\dfrac{36}{35}\)

\(a,\)\(\left(5x-15\right)\div5=0\)

\(\Rightarrow x-3=0\)

\(\Rightarrow x=3\)

\(b,\)\(84-4\left(2x+1\right)=48\)

\(\Rightarrow84-8x-4=48\)

\(\Rightarrow8x=32\)

\(\Rightarrow x=4\)

a) x = 4/5 : 5/7

x = 28/25

b) 3/5 : x = 5/6 - 2/5

3/5 : x = 13/30

x = 3/5 : 13/30

x = 18/13

\(a,\Leftrightarrow x=\dfrac{4}{5}:\dfrac{5}{7}\)

\(\Leftrightarrow x=\dfrac{28}{25}\)

\(b,\Leftrightarrow\dfrac{3}{5}:x=\dfrac{13}{30}\)

\(\Leftrightarrow x=\dfrac{18}{13}\)

\(a,x+\dfrac{2}{5}=\dfrac{1}{2}\)

\(x=\dfrac{1}{2}-\dfrac{2}{5}\)

\(x=\dfrac{5}{10}-\dfrac{4}{10}\)

\(\Rightarrow x=....\)

\(a,x+\dfrac{2}{5}=\dfrac{1}{2}\\ \Rightarrow x=\dfrac{1}{2}-\dfrac{2}{5}=\dfrac{5-4}{10}=\dfrac{1}{10}\)

\(b,x-\dfrac{2}{5}=\dfrac{1}{7}\\ \Rightarrow x=\dfrac{1}{7}+\dfrac{2}{5}=\dfrac{5+14}{35}=\dfrac{19}{35}\)

\(c,x\cdot\dfrac{3}{4}=\dfrac{9}{20}\\ \Rightarrow x=\dfrac{9}{20}:\dfrac{3}{4}=\dfrac{9}{20}\cdot\dfrac{4}{3}=\dfrac{3\cdot1}{5\cdot1}=\dfrac{3}{5}\)

\(d,x:\dfrac{1}{7}=14\\ \Rightarrow x=14\cdot\dfrac{1}{7}=\dfrac{14}{7}=2\)

\(e,\dfrac{2}{3}-x=\dfrac{1}{5}\\ \Rightarrow x=\dfrac{2}{3}-\dfrac{1}{5}=\dfrac{10-3}{15}=\dfrac{7}{15}\)

\(f,\dfrac{4}{15}:x=\dfrac{12}{25}\\ \Rightarrow x=\dfrac{4}{15}:\dfrac{12}{25}=\dfrac{4}{15}\cdot\dfrac{25}{12}=\dfrac{1\cdot5}{3\cdot3}=\dfrac{5}{9}\)

\(x\) \(\times\) \(\dfrac{4}{5}\) = 15

\(x\)         = 15 : \(\dfrac{4}{5}\)

\(x\)         = \(\dfrac{75}{4}\) 

\(\dfrac{12}{25}\) \(\times\) \(x\) = \(\dfrac{4}{10}\)

          \(x\) = \(\dfrac{4}{10}\) : \(\dfrac{12}{25}\)

          \(x\) = \(\dfrac{5}{6}\) 

\(x\times2\dfrac{2}{3}=3\dfrac{4}{8}+6\dfrac{5}{12}\) 

\(x\times\dfrac{8}{3}=\dfrac{28}{8}+\dfrac{77}{12}\) 

\(x\times\dfrac{8}{3}=\dfrac{119}{12}\) 

\(x=\dfrac{119}{12}:\dfrac{8}{3}\) 

\(x=\dfrac{119}{32}\)

\(x\) x 8/3 = 7/2 + 77/12

\(x\) x 8/3 = 119/12

\(x\) = 119/12 : 8/3 

x = 119/32

4 + x = 7 - 2x

4 + x + 2x = 7

4 + 3x = 7

3x = 7 - 4

3x = 3

x = 3 : 3

x =1

5 + x = 8,3 + 4,7 - x

5 + x + x = 8,3 + 4,7

5 + 2x = 13

2x = 13 -  5

2x = 8

x = 8 : 2

x= 4