`48/84=(48:2)/(84:2)=24/42=(24:2)/(42:2)=12/21=(12:3)/(21:3)=4/7`

\(\dfrac{48}{84}=\dfrac{24}{42}=\dfrac{12}{21}=\dfrac{4}{7}\)

a.(x + 125) : 312 = 14 (dư 300)

x+125 = 14 x 312 + 300

x+125 = 4 668

x= 4 668 - 125

x=4 543

b.(x-234):23=128 (dư 21)

    x-234=128 x 23 + 21

    x-234=2 965

    x=2 965 + 234

    x=3 199

c.(x-24) x 2020=2020x33

   (x-24)x2020=66 660

    x-24=66 660:2020

    x-24=33

    x=33+24

    x=57

d.5 x (x+2019)=3800 x 5

   5 x (x+2019)=19 000

   x+2019=19000:5

   x+2019=3800

   x=3800-2019

   x=1 781

HẾT=)))

a.(x + 125) : 312 = 14 (dư 300)

x+125 = 14 x 312 + 300

x+125 = 4 668

x= 4 668 - 125

x=4 543

b.(x-234):23=128 (dư 21)

    x-234=128 x 23 + 21

    x-234=2 965

    x=2 965 + 234

    x=3 199

c.(x-24) x 2020=2020x33

   (x-24)x2020=66 660

    x-24=66 660:2020

    x-24=33

    x=33+24

    x=57

d.5 x (x+2019)=3800 x 5

   5 x (x+2019)=19 000

   x+2019=19000:5

   x+2019=3800

   x=3800-2019

   x=1 781

HẾT=)))

\(a.16307:y=45\left(dư\text{ }17\right)\)\(\Leftrightarrow45\text{×}y+17=16307\)

\(45\text{×}y=16307-17\)

\(45\text{×}y=16290\)

\(y=16290:45\)

\(y=362\)

\(b.y\text{×}52+y\text{×}48=36700\)

\(y\text{×}\left(52+48\right)=36700\)

\(y\text{×}100=36700\)

\(y=36700:100\)

\(y=367\)

a) 16307= 45*y+17 -> y=(16307-17)/45= 362
b) y*(52+48)= 36700 -> y=36700/100= 367

muốn tìm y ta lấy nhân chéo chia đối 

ta đổi 24/48 = 1/2 sau đó ta 1 x 5 : 2 = 2,5 nhé 

124 : x + 24 : x - 48: x = 25

( 124 + 24 - 48 ) : x = 25

100 : x = 25

x = 4

\(124\div x+24\div x-48\div x=25\)

\(\Leftrightarrow\left(124+24-48\right)\div x=25\)

\(\Leftrightarrow100\div x=25\)

\(\Leftrightarrow x=100\div25=4\)

\(\dfrac{4}{x}=\dfrac{y}{21}=\dfrac{28}{49}\\ \Rightarrow\dfrac{4}{x}=\dfrac{y}{21}=\dfrac{4}{7}\)

\(\dfrac{4}{x}=\dfrac{4}{7}\Rightarrow x=7\)

\(\dfrac{y}{21}=\dfrac{4}{7}\Rightarrow\dfrac{y}{21}=\dfrac{12}{21}\Rightarrow y=12\)

thank you nhé 

vì 4/x=28/49 nên x.28=4.49=196 ( dấu . là dấu nhân , vì sao x.28=4.49 cấp 2 học )

x=196:28=7

vì y/21=28/49 nên y.49=21.28=588

y=588:49=12

\(\frac{4}{x}=\frac{y}{21}=\frac{28}{49}\)

\(\Rightarrow\frac{4}{x}=\frac{12}{21}=\frac{28}{49}\)

\(\Rightarrow\frac{4}{7}=\frac{12}{21}=\frac{28}{49}\)

Vậy x = 7 , y = 12

Cách làm:

y = \(\frac{28\times21}{49}=12\)

x = \(\frac{21\times4}{12}=7\)

Công thức : nhân chéo

#Hok tốt !

[(2+y)x81-3530]=714:21

[(2+y)x81-3530]=34

(2+y)x81=34+3530

(2+y)x81=3564

2+y=3564:81

2+y=44

y=44-2

y=42

[ y*81 +2*81-3530 ] *21 =714

y*81 +2*81-3530=714:21

81*(y+2)-3530=34

81*(y+2)=34+3530

81*(y+2)=3564

y+2=3564:81

y+2=44

y=44-2

y=42