42/25 * x/5 = 6/5

x/5 = 5/7

x= 25/7

\(a.\dfrac{12+x}{42}=\dfrac{35}{42}\Leftrightarrow12+x=35\Leftrightarrow x=23\)

\(b.\dfrac{25-x}{40}=\dfrac{3}{8}\Leftrightarrow\dfrac{25-x}{40}=\dfrac{15}{40}\Leftrightarrow25-x=15\Leftrightarrow x=10\)

\(c.\dfrac{13+x}{20}=\dfrac{3}{4}\Leftrightarrow\dfrac{13+x}{20}=\dfrac{15}{20}\Leftrightarrow13+x=15\Leftrightarrow x=2\)

\(d.\dfrac{23-x}{25}=\dfrac{20}{25}\Leftrightarrow23-x=20\Leftrightarrow x=3\)

\(\frac{42}{25}\div\frac{x}{5}=\frac{6}{5}\)

\(\frac{x}{5}=\frac{42}{25}\div\frac{6}{5}\)

\(\frac{x}{5}=\frac{42}{25}\times\frac{5}{6}\)

\(\frac{x}{5}=\frac{7}{5}\)

\(x=7\)

\(\frac{42}{25}:\frac{x}{5}=\frac{6}{5}\) 

\(\frac{x}{5}=\frac{42}{25}:\frac{6}{5}\) 

\(\frac{x}{5}=\frac{42}{25}.\frac{5}{6}\) 

\(\frac{x}{5}=\frac{7}{5}\) 

\(\Rightarrow x=7\) 

Vậy \(x=7\)

\(\frac{42}{25}:\frac{x}{5}=\frac{6}{5}\)

\(\frac{x}{5}=\frac{42}{25}:\frac{6}{5}\)

\(\frac{x}{5}=\frac{42}{25}.\frac{5}{6}\)

\(\frac{x}{5}=\frac{7}{5}\)

\(\Rightarrow x:5=\frac{7}{5}\)

\(\Rightarrow x=\frac{7}{5}.5\)

\(\Rightarrow x=7\)

b: \(\Leftrightarrow25-x=30\)

hay x=-5

\(a,12+\frac{x}{42}=\frac{5}{6}\)

\(\Rightarrow\frac{x}{42}=\frac{5}{6}-12\)

\(\Rightarrow\frac{x}{42}=\frac{-67}{6}\)

\(\Leftrightarrow6x=42\times(-67)\)

\(\Leftrightarrow6x=-2814\)

\(\Leftrightarrow x=-2814\div6=-469\)

Vậy x = -469

\(b,25-\frac{x}{40}=\frac{3}{8}\)

\(\frac{x}{40}=25-\frac{3}{8}\)

\(\frac{x}{40}=\frac{197}{8}\)

\(\Leftrightarrow8x=40\times197\)

\(\Leftrightarrow8x=7880\)

\(\Leftrightarrow x=7880\div8=985\)

Vậy x = 985

Chúc bạn học tốt

a, \(12+\frac{x}{42}=\frac{5}{6}\Leftrightarrow\frac{x}{42}=-\frac{67}{6}\Leftrightarrow\frac{x}{42}=\frac{-469}{42}\)

Khử mẫu đi ta đc : \(x=-469\)

b, \(25-\frac{x}{40}=\frac{3}{8}\Leftrightarrow\frac{x}{40}=\frac{197}{8}\Leftrightarrow\frac{x}{40}=\frac{985}{40}\)

Khử mẫu đi ta đc : \(x=985\)

x/42=5/6-12

x/42=-67/6

x.6=42.-67

x.6=-2814

x=-2814:6

x=-469

a: =23 giờ 63 phút

=24 giờ 3 phút

b: =606 giờ-105 giờ=501 giờ

c: =15 giờ 135 phút

=17 giờ 15 phút

d: =2550 giây:5

=510 giây

= 0

( 5 + 10 + 15 + 20 + 25 + 30 + 35 ) x ( 42 - 6 x 7 ) 

= ( 5 + 10 + 15 + 20 + 25 + 30 + 35 ) x ( 42 - 42 )

= ( 5 + 10 + 15 + 20 + 25 + 30 + 35 ) x 0

= 0

=0 nha