1) 1/(R2-4) = (2/15)+(1/30) =5/30 = 1/6. R2-4 = 6 Ом.
Rобщ = 5,4+6+6+10,8 = 28,2 Ом.
2) 1/(R2-3) = (1/48)+(1/16) = 4/48 = 1/12. R2-3 = 12 Ом.
1/(R4-7) = (1/10)+(2/25)+(1/50) = 10/50. R4-7 = 5 Ом.
Rобщ = 10+12+5+12 = 39 Ом.
3) 1/(R1-4) = (2/15)+(2/30) = 6/30 = 1/5. R1-4 = 5 Ом.
1/(R6-7) = (1/12)+(1/24) = 3/24 = 1/8. R6-7 = 8 Ом..
Rобщ =5+10+8+8 = 31 Ом.