Proporzioni

(1/8+3/4+1/2+5/8)/(3²-2-x)=(26/15*10/13/(1/2))/x

(1/8+6/8+4/8+5/8)/(7-x)=(2/3*2/1/(1/2))/x

2/(7-x)=4/3(1/2)/x

1/(7-x)=2/3(1/2):x

1/(7-x)=4/(3x)   ---> 3x/4 =7-x

28 = 7x  --->   x =4

@amerika

(120/8-x) : x = 2/5 10

(15-x) : x = 2/5 : (6/10)

si fa il prodotto in croce

(15-x)*6/10 = 2x/5

9-6x/10 = 2x/5

9 = 2x/5+6x/10

9 = (4x+6x)/10

9 = 10x/10 

x = 9  

(1+6+4+5)/8 : (7-x) = (20/15*2) : x

2 : (7-x) = 8/3 : x 

si fa il prodotto in croce

2x = 56/3-8x/3

2x+8x/3 = 56/3

14x/3 = 56/3

14x = 56

x = 56/14

x = 4 

326)

$\small \left(\dfrac{3}{28}·\dfrac{7}{2}·40-x\right) : x = \dfrac{2}{5} : \left(\dfrac{7}{10}-\dfrac{2}{5}+\dfrac{3}{10}\right)$

$\small \left(\dfrac{3}{\cancel{28}_4}·\dfrac{\cancel7^1}{\cancel2_1}·\cancel{40}^{20}-x\right) : x = \dfrac{2}{5} : \left(\dfrac{7}{10}-\dfrac{2}{5}+\dfrac{3}{10}\right)$

$\small \left(\dfrac{3}{4}·\dfrac{1}{1}·20-x\right) : x = \dfrac{2}{5} : \left(\dfrac{7-4+3}{10}\right)$

$\small \left(\dfrac{3}{\cancel4_1}·\cancel{20}^5-x\right) : x = \dfrac{2}{5} : \dfrac{6}{10}$

$\small \left(3·5-x\right) : x = \dfrac{2}{5} : \dfrac{\cancel6^3}{\cancel{10}_5}$

$\small \left(15-x\right) : x = \dfrac{2}{5} : \dfrac{3}{5}$

applica il metodo del comporre:

$\small \left(15-x+x\right) : x = \left(\dfrac{2}{5}+\dfrac{3}{5}\right) : \dfrac{3}{5}$

$\small \left(15-\cancel{x}+\cancel{x}\right) : x = \dfrac{\cancel5^1}{\cancel5_1} : \dfrac{3}{5}$

$\small 15 : x = 1 : \dfrac{3}{5}$

$\small x = \cancel{15}^3×\dfrac{3}{\cancel5_1} : 1$

$\small x = 3×3 = 9$

327)

$\small \left(\dfrac{1}{8}+\dfrac{3}{4}+\dfrac{1}{2}+\dfrac{5}{8}\right) : \left(3^2-2-x\right) = \left(\dfrac{26}{15}·\dfrac{10}{13} : \dfrac{1}{2}\right) : x$

$\small \left(\dfrac{1+6+4+5}{8}\right) : \left(9-2-x\right) = \left(\dfrac{\cancel{26}^2}{\cancel{15}_3}·\dfrac{\cancel{10}^2}{\cancel{13}_1}·2\right) : x$

$\small \dfrac{16}{8} : \left(7-x\right) = \left(\dfrac{2}{3}·2·2\right) : x$

$\small \dfrac{\cancel{16}^2}{\cancel8_1} : \left(7-x\right) = \left(\dfrac{2}{3}·4\right) : x$

$\small 2 : \left(7-x\right) = \dfrac{8}{3} : x$

$\small x = \dfrac{\dfrac{8}{3}\left(7-x\right)}{2}$

moltiplica tutto per 2:

$\small 2x = \dfrac{8}{3}\left(7-x\right)$

$\small 2x = \dfrac{56}{3}-\dfrac{8}{3}x$

moltiplica tutto per 3:

$\small 6x = 56-8x$

$\small 6x+8x = 56$

$\small 14x = 56$

dividi per 14 ambo le parti per isolare l'incognita:

$\small \dfrac{\cancel{14}x}{\cancel{14}} = \dfrac{56}{14}$

$\small x = 4$