a) 39 - 15 : (x : 2) = 24

15 : x. 2 = 39 - 24

30 : x = 15

x = 30 : 15

x = 2

Vậy x = 2

b) 720 : [41 − (2x − 5)] = 8,5

41 - 2x + 5 = 720 : 8,5

41 - 2x + 5 = $\frac{1440}{17}$ 

46 - 2x = $\frac{1440}{17}$

2x = 46 - $\frac{1440}{17}$

2x = $\frac{-658}{17}$ 

x = $\frac{-658}{17}$ : 2

x = $\frac{-329}{17}$

Vậy x = $\frac{-329}{17}$

c) (5x - 1). 8 - 6 = 106

40x - 8 = 106 + 6

40x = 106 + 6 + 8

40x = 120

x = 120 : 40

x = 3 

Vậy x = 3 

d) (x + 1) + (x + 2) + (x + 3) + ... + (x + 10) = 255

(x + 10 + x + 1). [(x + 10 - x - 1) : 1 + 1] : 2 = 255

(2x + 11). 10 = 255. 2

20x + 110 = 510

20x = 510 - 110

20x = 400

x = 400 : 20

x = 20

Vậy x = 20

e) 112 + (42 - x) = 100

42 - x = 100 - 112

42 - x = -12

x = 42 + 12

x = 54

Vậy x = 54

a) $39-15:(x:2)=24$

$15:(x:2)=39-24=15$

$x:2=15:15=1$

$x=1.2=2$

b) $720:[41-(2x-5)]=8,5$

$41-(2x-5)=720:8,5=\dfrac{1440}{17}$

$2x-5=41-\dfrac{1440}{17}=-\dfrac{743}{17}$

$2x=-\dfrac{743}{17}+5=-\dfrac{658}{17}$

$x=-\dfrac{658}{17}:2=-\dfrac{658}{17}.\dfrac{1}{2}=-\dfrac{658}{34}=-\dfrac{329}{17}$

c) $(5x-1).8-6=106$

$(5x-1).8=112$

$5x-1=112:8=14$

$5x=14+1=15$

$x=15:5=3$

d) $(x+1)+(x+2)+(x+3)+...+(x+10)=255$

$x+1+x+2+x+3+...+x+10=255$

$10x+(1+2+...+10)=255$

$10x+55=255$

$10x=255-55=200$

$x=200:10=20$

e) $112+(42-x)=100$

$42-x=100-112=-12$

$x=42+12=54$