a) x = 100.           

b) x = 28.            

c) x = 0.               

d) x = 0.

Bài cô t vừa giao nè

\(a,x:\left(111-99\right)=17\cdot5\\ \Leftrightarrow x:12=85\\ \Leftrightarrow x=1020\\ b,\left(509+335\right):x=840:35\\ \Leftrightarrow844:x=24\\ \Leftrightarrow x=\dfrac{211}{6}\\ c,\left(x+1\right)+\left(x+2\right)+...+\left(x+28\right)=462\\ \Leftrightarrow28x+406=462\\ \Leftrightarrow28x=56\\ \Leftrightarrow x=2\)

d, 890 : x = 35 dư 15

\(\Rightarrow35x+15=890\\ \Leftrightarrow35x=875\\ \Leftrightarrow x=25\)

ddddddddddddddddddddddđẽ

(x + 3) + (x + 7) + (x + 11) + ... + (x + 79) = 860

=> x + 3 + x + 7 + x + 11 + ... + x + 79 = 860

=> (x + x + x + ... + x) + (3 + 7 + 11 + ... + 79) = 860

=> 20x + (79 + 3).20 : 2 = 860

=> 20x + 82.20 : 2 = 860

=> 20x + 82.10 = 860

=> 20x + 820 = 860

=> 20x = 40

=> x = 2

vậy_

#)Giải :

\(\left(x+3\right)+\left(x+7\right)+...+\left(x+79\right)=860\)

\(\left(x+x+...+x\right)+\left(3+7+...+79\right)=860\)(trong mỗi ngoặc có 20 số hạng)

\(x\times20+\frac{\left(79+3\right)\times20}{2}=860\)

\(x\times20+820=860\)

\(x\times20=860-820\)

\(x\times20=40\)

\(x=40\div20\)

\(x=2\)

Lời giải 

\(\left(x+x+...+x\right)+\left(3+7+...+79\right)=860\)

\(x.20+\frac{\left(79+3\right).20}{2}=860\)

\(x.20+820=860\)

\(x.20=40\)

\(x=2\)

$a)(x-25)-175=0$

$\to x-25=0+175=175$

$\to x=175+25$

$\to x=200$

$b)485-(6.x+60)=5$

$\to 6.x+60=485-5=480$

$\to 6.x=480-60=420$

$\to x=420:6=70$

c) ( x - 31 ) - 68 = 0

x - 31 - 68 = 0

x - 99 = 0

x - 99 +99 = 0 + 99

x = 0 + 99

x = 99

a)\(x=875-\left(50\times7\right)=875-350=525\)

b)\(x=504:14=36\)

a,525

b,36

c,69

d,14620

c)\(x=5760:96=60\)

d)\(x=860\times17=14620\)