c)\(\dfrac{3}{8}\times\dfrac{5}{8}+y=\dfrac{5}{4}\) 

   \(\dfrac{15}{64}+y=\dfrac{5}{4}\) 

           \(y=\dfrac{5}{4}-\dfrac{15}{64}\) 

           \(y=\dfrac{65}{64}\)

d, \(\dfrac{3}{8}+\dfrac{5}{8}\times y=\dfrac{5}{4}\) 

          \(\dfrac{5}{8}\times y=\dfrac{5}{4}-\dfrac{3}{8}\) 

          \(\dfrac{5}{8}\times y=\dfrac{7}{8}\) 

                 \(y=\dfrac{7}{8}:\dfrac{5}{8}\) 

                \(y=\dfrac{7}{5}\)

a) 3/4 x y = 9/10

y = 9/10 : 3/4

y = 6/5

b) 3/5 : y = 7/20

y = 3/5 : 7/20 

y = 12/7

e)\(\dfrac{1}{3}+\dfrac{2}{3}:x=\dfrac{5}{2}\) 

       \(\dfrac{2}{3}:x=\dfrac{5}{2}-\dfrac{1}{3}\) 

       \(\dfrac{2}{3}:x=\dfrac{13}{6}\) 

             \(x=\dfrac{2}{3}:\dfrac{13}{6}\) 

             \(x=\dfrac{4}{13}\)

x/y=3/4

=>x/3=y/4

=>x/15=y/20

y/z=5/7

=>y/5=z/7

=>y/20=z/28

=>x/15=y/20=z/28=(2x+3y-z)/(2*15+3*20-28)=186/62=3

=>x=45; y=60; z=84

\(d,\dfrac{6}{5}-3:\dfrac{15}{4}=\dfrac{6}{5}-3\times\dfrac{4}{15}=\dfrac{6}{5}-\dfrac{4}{5}=\dfrac{2}{5}\)

\(b,\dfrac{2}{5}+\dfrac{4}{5}:4=\dfrac{2}{5}+\dfrac{4}{5}\times\dfrac{1}{4}=\dfrac{2}{5}+\dfrac{1}{5}=\dfrac{3}{5}\)

\(a,\dfrac{5}{8}+\dfrac{4}{3}=\dfrac{15}{24}+\dfrac{32}{24}=\dfrac{47}{24}\)

a: Thay \(y=\dfrac{1}{3}\) vào (d3), ta được:

\(\dfrac{-2}{3}x+\dfrac{5}{3}=\dfrac{1}{3}\)

\(\Leftrightarrow x=2\)

Thay x=2 và \(y=\dfrac{1}{3}\) vào (d), ta được:

\(2\left(m-2\right)+m+7=\dfrac{1}{3}\)

\(\Leftrightarrow3m=\dfrac{1}{3}-3=\dfrac{-8}{3}\)

hay \(m=-\dfrac{8}{9}\)

Ta có :

\(\frac{x}{3}=\frac{y}{5}\)  (1)

\(\left(x+2\right)+\left(y+3\right)=21\)

\(\Leftrightarrow x+2+y+3=21\)

\(\Leftrightarrow x+y=16\)

\(\Leftrightarrow x=16-y\)

Thay x = 16 - y vào (1) ta được :

\(\frac{16-y}{3}=\frac{y}{5}\)

\(\Rightarrow5\left(16-y\right)=3y\)

\(\Leftrightarrow80-5y=3y\)

\(\Leftrightarrow80=8y\)

\(\Leftrightarrow y=10\)

\(\Rightarrow x=16-10=6\)

Vậy x = 6 và y = 10