a) \(20\times6\times5\times7\)
\(=\left(20\times5\right)\times6\times7\)
\(=100\times6\times7\)
\(=4200\)

b) \(16\times4\times5\times20\)
\(=16\times4\times100\)
\(=16\times400\)
\(=6400\)

a) (25x32) : 8           

= 25 x (4 x 8) : 8

= (25 x 4) x (8:8)

= 100 x 1

=100

b, 24 x 11 + 6 x 86 x 4 + 72

= 24 x 11 + (6 x 4) x 86 + 24 x 3

= 24 x 11 + 24 x 86 + 24 x 3

= 24 x (11+86+3)

= 24 x 100

= 2 400

\(\left(25\times32\right):8=25\times32:8=25\times\left(32:8\right)=25\times4=100\\ 24\times11+6\times86\times4+72\\ =24\times11+24\times86+24\times3\\ =24\times\left(11+86+3\right)\\ =24\times100=2400\\ 36\times5+9\times79\times4+72\times8\\ =36\times5+36\times79+36\times16\\ =36\times\left(5+79+16\right)\\ =36\times100=3600\\ \left(56\times125\right):7=56\times125:7=\left(56:7\right)\times125=8\times125=1000\)

b,21/17x13/14x56x3/42

=21/17x3/42x56x13/14

=21x3/17x42 x 56x13/14

=21x3/17x21x2 x 14x4x13/14

=3/17x2 x 4x13

=3/24 x 52=3x52/24

=3x52/3x8

=52/8=13/2

a, 6/7 x 16/15 x 7/6 x 21/32

=(6/7 x 7/6)x16/15x21/32

=16/15x21/32

=16x21/15x32

=16x3x7/3x5x16x2

=7/5x2=7/10

a, = \(\frac{11}{21}\)x \(\left(\frac{7}{4}+\frac{5}{4}\right)\)

   = \(\frac{11}{21}\)x 3

  = \(\frac{33}{21}\)

b) = \(\left(\frac{23}{14}-\frac{9}{14}\right)\)x \(\frac{6}{13}\)

    = 1 x \(\frac{6}{13}\)

    = \(\frac{6}{13}\)

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\(\frac{8}{15}\)x \(\frac{5}{16}\)= \(\frac{40}{240}\)= \(\frac{1}{6}\)

2 . ( chịu )

3 .          Giải 

Nếu gấp thừa số thứ nhất lên 2 lần thì tích sẽ gấp lên 2 lần

       Vậy  tích mới của hai số là : 

             \(\frac{3}{16}\)x 2 = \(\frac{6}{16}\)= \(\frac{3}{8}\)

4. Tính 

a) \(\frac{75}{55}\)x 4 x \(\frac{33}{10}\)                  b) \(\frac{49}{84}\)x \(\frac{16}{21}\)x 15 + 4                    c) ............................ ( phép này dễ tự tính )                                                                                   

= \(\frac{75}{55}\)x \(\frac{4}{1}\) x \(\frac{33}{10}\)           = \(\frac{784}{1764}\) = \(\frac{441}{196}\)x 15 + 4                                                                                                  

= \(\frac{300}{55}\)= \(\frac{60}{11}\)x \(\frac{33}{10}\)      = .............( tự tính nốt nhé )                                                                                                                                                 

= \(\frac{1980}{110}\)= \(18\)                                                                                                                                                                     

cảm ơn rất nhiều

1,

a,=4/7:4/9.3

=9/7.3

=27/7

b,=(7/11+4/11).5/6

=1.5/6

=5/6

`(6/7-2/7):4/9xx3=4/7xx9/4xx3=[4xx9xx3]/[7xx4]=27/7`

`7/11xx5/6+4/11xx5/6=(7/11+4/11)xx5/6=11/11xx5/6=5/6`

`4/5 xx 3/7 + 4/5 xx 6/7 - 4/5 xx 4/14`

`= 4/5 xx ( 3/7 + 6/7-4/14)`

`= 4/5 xx 1`

`= 4/5`