Giải thích các bước giải:

Bài 6:

$b)\dfrac{7}{8}-\dfrac{7}{12}$

$=\dfrac{7×3}{8×3}-\dfrac{7×2}{12×2}$

$=\dfrac{21}{24}-\dfrac{14}{24}$

$=\dfrac{7}{24}$

$b)\dfrac{9}{10}-\dfrac{3}{7}$

$=\dfrac{9×7}{10×7}-\dfrac{3×10}{7×10}$

$=\dfrac{63}{70}-\dfrac{30}{70}$

$=\dfrac{33}{70}$

Bài 7:

$a)\dfrac{39}{50}-y=\dfrac{3}{10}$

$⇒\dfrac{39}{50}-\dfrac{3}{10}=y$

$⇒y=\dfrac{12}{25}$

$b)\dfrac{7}{35}+y=\dfrac{4}{5}$

$⇒y=\dfrac{4}{5}-\dfrac{7}{35}$

$⇒y=\dfrac{3}{5}$

Bài 8:

$\dfrac{3}{7}-\dfrac{2}{21}$

$=\dfrac{3×3}{7×3}-\dfrac{2}{21}$

$=\dfrac{9}{21}-\dfrac{2}{21}$

$=\dfrac{7}{21}$

$=\dfrac{1}{3}$

$b)\dfrac{10}{9}-\dfrac{5}{12}$

$=\dfrac{10×4}{9×4}-\dfrac{5×3}{12×3}$

$=\dfrac{40}{36}-\dfrac{15}{36}$

$=\dfrac{25}{36}$

$c)\dfrac{27}{10}-2$

$=\dfrac{27}{10}-\dfrac{2×10}{1×10}$

$=\dfrac{27}{10}-\dfrac{20}{10}$

$=\dfrac{7}{10}$

6.

b) 7/8 - 7/12= 21/24 - 14/24 = 21-14/24 = 7/24 

c) 9/10 - 3/7 = 63/70 - 30/70 = 63-30/70 = 33/70

7.

a) 39/50 - y = 3/10

⇒-y = 3/10 - 39/50

⇒-y= -12/25

⇒y=12/25

 Vậy y=12/25

b) 7/35 + y = 4/5

⇒y=4/5 - 7/35

⇒y= 3/5

   Vậy y= 3/5

8.

a) 3/7 - 2/21= 9/21 - 2/21 = 7/21 = 1/3

b) 10/9 - 5/12 = 40/36 - 15/36 = 25/36

c) 27/10 - 2= 27/10 - 20/10= 7/10