resistenze in serie ed in parallelo

(R parallelo 1) + (R parallelo 2) = R equivalente; 

(1/R1 + 1/R2)^-1 + ( 1/R3 + 1/R4 + 1/R5)^-1 = R equivalente;

(1/20 + 1/30)^-1  + (1/70 + 1/30 + 1/R5)^-1 = 20,4 Ω;

(3/60 + 2/60)^-1 + ( 3/210 + 7/210 + 1/R5)^-1 = 20,4;

(5/60)^-1 + (10/210 + 1/R5)^-1 = 20,4 ;

(1/12)^-1 + (1/21 + 1/R5)^-1 = 20,4 ;

12  +  (1/21 + 1/R5)^-1 = 20,4 ;

(1/21 + 1/R5)^-1 = 20,4 - 12;

[(R5 + 21) / (21 * R5)]^-1 = 20,4 - 12;

21 * R5 / (R5 + 21) = 8,4;

21 R5 = 8,4 * (R5 + 21);

21 R5 = 8,4 R5 + 176,4;

21 R5 - 8,4 R5 = 176,4;

12,6 R5 = 176,4;

R5 = 176,4 / 12,6 = 14 Ω.

Ciao @alessiamoro

R1//R2+Rp =  20,4 ohm

(20*30)/(20+30) = 600/50 = 60/5 = 12 ohm

Rp = 20,4-12 = 8,4 ohm 

Rp = 70 // 30 // R5 

Rp' = 70//30 = 70*30/(70+30) = 2100/100 = 21 ohm 

Rp = 8,4 = (Rp'//R5) 

8,4 = 21*R5/(21+R5)

176,4 = R5(21-8,4)

R5 = 176,4/12,6 = 14,0 ohm 

R+R/2 = R/3+700

3R/2-R/3 = 700

(9R-2R)/6 = 700

7R = 700*6

R = 600 ohm

dò ad R il valore unitario

1) 1+2+3 = 6 

2) 1//2//3 = 1/(1+1/2+1/3) = 0,(54)

3) 1+2//3 = 1+6/5 = 11/5 = 2,20

4) 2+1//3 = 2+3/4 = 11/4 = 2,75

5) 3+1//2 = 3+2/3 = 11/3 = 3,(6)

6) (1+2)//3 = 3//3 = 3/2 = 1,50

7) (1+3)//2 = 4//2 = 4/3 = 1,(3)

8) (2+3)//1 = 5//1 = 5/6 = 0,8(3)

0,(54) ; 0,8(3) ; 1,(3) ; 1,5 ; 2,20 ; 2,75 ; 3,6 ; 6