# Please help me solve this combo circuit (series-parallel) Check the picture. It has 4 resistors.  Given  I total = 0.4A R1 = 10 ohms R3 = 20 ohms V4 = 6V

crmhaske | College Teacher | (Level 3) Associate Educator

Posted on

For series resistors, the equivalent resistance is the summation of the resistances. Therefore:

`R_5=R_2+R_3`

For parallel resistors, the equivalent resistance is 1 over the sumation of 1 over the resistances:

`1/R_6=1/R_5+1/R_4`

`R_6=(R_5R_4)/(R_5+R_4)`

`=(R_4(R_2+R_3))/(R_2+R_3+R_4)`

We know that the voltage across R6 is V4. Therefore, according to Ohm's Law:

`I_T = V_4/R_6`

`R_6=V_4/I_T`

`(R_4(R_2+R_3))/(R_2+R_3+R_4) = V_4/I_T`

Now, substitute what we know:

`(R_4(R_2+20))/(20+R_2+R_4)=6/0.4=15`

We don't have enough information to solve any further than that; therefore, `R_2 ` and `R_4` are any such values that satisfy the above equation.

Sources:

pixelgera | High School Teacher | eNotes Newbie

Posted on

R2 AND R4 CAN HAVE A SET OF ANY VALUES

eg:- R2 =40 ohm, R4=20 ohm or other values satisfying relationship:-

R4( R2+20)

-------------  =  15

(R2+R4 +20)

hope this helps.

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