Sos proporzioni

7/8*4/3*x+x = 2/5

7x/6+x = 2/5

13x/6 = 2/5

x = 12/(5*13) = 12/65

check : 

7/6 = (2/5-12/65)*65/12

7/6 = (26-12)/65*65/12 

7/6 = 14/12 

7/6 = 7/6 ....OK !!

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$\small \dfrac{7}{8} : \dfrac{3}{4} = \left(\dfrac{2}{5}-x\right) : x$

qui applichi la proprietà del comporre:

$\small \left(\dfrac{7}{8}+\dfrac{3}{4}\right) : \dfrac{3}{4} = \left(\dfrac{2}{5}-\cancel{x}+\cancel{x}\right) : x$

$\small \left(\dfrac{7+6}{8}\right) : \dfrac{3}{4} = \dfrac{2}{5} : x$

$\small \dfrac{13}{8} : \dfrac{3}{4} = \dfrac{2}{5} : x$

$\small x= \dfrac{3}{\cancel4_2}·\dfrac{\cancel2^1}{5} : \dfrac{13}{8}$

$\small x= \dfrac{3}{\cancel2_1}·\dfrac{1}{5}·\dfrac{\cancel8^4}{13}$

$\small x= \dfrac{3}{1}·\dfrac{1}{5}·\dfrac{4}{13}$

$\small x= \dfrac{12}{65}$ $\small  \quad(= 0,1846153846).$