a) 4/5 (x - 30%) = 1/4

=> x - 30% = 1/4 : 4/5

=> x - 3/10 = 5/16

=> x = 5/16 + 3/10

=> x = 49/80

Vậy x = 49/80

b) (x - 3/7)(x + 3 1/11) = 0

=> (x-3/7)(x + 34/11)= 0

=> \(\left[ \begin{array}{l}x-\dfrac{3}{7}=0\\x + \dfrac{34}{11}=0\end{array} \right.\)

=> \(\left[ \begin{array}{l}x=\dfrac{3}{7}\\x=\dfrac{-34}{11}\end{array} \right.\) 

Vậy x in {3/7 ; -34/11}

a) 4/5(x - 30%) = 1/4

$$\Rightarrow$$ x - 30/100 = 1/4 : 4/5

$$\Rightarrow$$ x - 3/10 = 1/4 . 5/4

$$\Rightarrow$$ x - 3/10 = 5/16

$$\Rightarrow$$ x = 5/16 + 3/10

$$\Rightarrow$$ x = 25/80 + 24/80

$$\Rightarrow$$ x = 49/80

Vậy x = 49/80

b) (x - 3/7)(x + 3 1/11) = 0

$$\Rightarrow$$ (x - 3/7)(x + 34/11) = 0

$$\Rightarrow$$ $$\left[\begin{matrix} x - \dfrac{3}{7} = 0\\ x + \dfrac{34}{11} = 0\end{matrix}\right.$$

$$\Rightarrow$$ $$\left[\begin{matrix} x = \dfrac{3}{7}\\ x = -\dfrac{34}{11}\end{matrix}\right.$$

Vậy x = 3/7 hoặc x = -34/11