Lời giải chi tiết :

Bài 1 : 

$\frac{3}{8}$  = $\frac{3X125}{8X125}$ = $\frac{375}{1000}$                            $\frac{7}{4}$= $\frac{7X25}{4X25}$ = $\frac{175}{100}$

$\frac{3}{2}$ =$\frac{3X5}{2X5}$ =$\frac{15}{10}$

$\frac{6}{25}$ = $\frac{6X4}{25X4}$ = $\frac{24}{100}$

$\frac{400}{4000}$ = $\frac{400 : 400}{4000:400}$ = $\frac{1}{10}$

$\frac{6}{300}$ = $\frac{6:3}{300:3}$ = $\frac{2}{100}$              

Bài 2 : 

2$\frac{3}{4}$ = $\frac{11}{4}$              4$\frac{7}{7}$ = $\frac{35}{7}$ = 5 

3$\frac{1}{5}$ = $\frac{16}{5}$              5$\frac{7}{9}$ = $\frac{52}{9}$ 

Bài 3 : 

a ) $\frac{3}{4}$ + 7$\frac{5}{6}$         b ) 12$\frac{2}{3}$ - 6$\frac{4}{5}$ 

=  $\frac{3}{4}$ + $\frac{47}{6}$             = $\frac{38}{3}$ - $\frac{34}{5}$ 

=      $\frac{103}{12}$                  = $\frac{88}{15}$ 

c ) 4$\frac{1}{6}$ x 7$\frac{9}{15}$           d ) 11$\frac{3}{8}$ : 8$\frac{5}{22}$ 

= $\frac{25}{6}$ x $\frac{38}{5}$                = $\frac{91}{8}$ : $\frac{21}{2}$ 

= $\frac{95}{3}$                        = $\frac{91}{8}$ x $\frac{2}{21}$ 

                               = $\frac{13}{12}$ 

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Bài `1 :`

`3/8 = ( 3 xx 125 )/( 8 xx 125 ) = 375/1000`

`7/4 = ( 7 xx 25 )/( 4 xx 25 ) = 175/100`

`3/2 = ( 3 xx 5 )/( 2 xx 5 ) = 15/10`

`6/25 = ( 6 xx 4 )/( 25 xx 4 ) = 24/100`

`400/4000 = ( 400 : 400 )/( 4000 : 400 ) = 1/10`

`6/300 = ( 6 : 3 )/( 300 : 3 ) = 2/100`

``

Bài `2 :`

`2 3/4 = 11/4`

`3 1/5 = 16/5`

`4 7/7 = 35/7 = 5`

`5 7/9 = 52/9`

``

Bài `3 :`

`a)` `3/4 + 7 5/6`

`= 3/4 + 47/6`

`= 103/12`

`b)` `12 2/3 - 6 4/5`

`= 38/3 - 34/5`

`= 88/15`

`c)` `4 1/6 xx 7 9/15`

`= 25/6 xx 38/5`

`= 95/3`

`d)` `11 3/8 : 8 5/22`

`= 91/8 : 21/2`

`= 91/8 xx 2/21`

`= 13/12`