a) X + 15 = 20

       X = 20 – 15

       X = 5

b) X – 18 = 16

       X = 16 + 18

       X = 34

c) 72 – x = 50

       X = 72 -50

       X =22

d) X x 7 = 42

       X= 42 : 7

       X = 6

e) X : 7 = 5

       X = 5 x 7

       X = 35

f) 49 : x = 7

       X = 49: 7

       X = 7

câu a là 7x là nhân hay x trong 7x là x

Giúp mình nha, mình đang cần gấp. Cho mik cảm ơn trước nha

Có \(\frac{x}{-28}\)=\(\frac{42}{49}\)

<=> x.49=-28.42

<=> x.49=-1176

<=> x     =-1176/49

<=>x      =-24

\(\frac{x}{-28}=\frac{42}{49}\)

=> \(\frac{x}{-28}=\frac{6}{7}\)

=> \(\frac{x:\left(-4\right)}{-28:\left(-4\right)}=\frac{6}{7}\)

=> \(x:\left(-4\right)=6\)

=> \(x=-24\)

a) 142 x 12 + 42 x 18

= 142 x (12 +18)

= 142 x 30 = 4260

b) 49 x 365 - 39 x 365

= (49-39) x 365

= 10 x 365 = 3650

c) 4 x 18 x 25

= 18 x (4 x 25)

= 10 x 100 = 1800

Đáp án D

Lời giải:
$200-18:(42:3\times x-13)-28=154$

$200-28-18:(42:3\times x-13)=154$

$172-18:(42:3\times x-13)=154$

$18:(42:3\times x-13)=172-154=18$

$42:3\times x-13=18:18=1$

$14\times x-13=1$

$14\times x=13+1=14$

$x=14:14=1$

\(200-18:\left(42:3\times x-13\right)-28=154\)

\(200-18:\left(42:3\times x-13\right)=182\)

\(18:\left(42:3\times x-13\right)=200-182\)

\(18:\left(42:3\times x-13\right)=18\)

\(42:3\times x-13=18:18\)

\(42:3\times x-13=1\)

\(42:3\times x=1+13\)

\(42:3\times x=14\)

\(14\times x=14\)

\(x=14:14\)

\(x=1\)

Ta có: \(200-18:\left(42:3x-13\right)-28=154\)

\(\Leftrightarrow172-18:\left(42:3x-13\right)=154\)

\(\Leftrightarrow18:\left(42:3x-13\right)=18\)

\(\Leftrightarrow42:3x-13=1\)

\(\Leftrightarrow42:3x=14\)

\(\Leftrightarrow3x=3\)

hay x=1

53/18 nha 

A

22

a