Two lamps can be connected to a battery in a circuit in series or in parallel. You can calculate the total resistance RT (t is down from r) in a circuit if you know the resistance in each lamp. Resistance is measured in ohms. For a circuit connected in series, RT = R1 + R2. For a circuit connected in parallel, 1/RT = 1/R1 + 1/R2.

Question

a. The lamps are connected in parallel. RT = 12 ohms. Find R2. R1 is 30 ohms.

Can you show me how to solve 1/12 = 1/30 + 1/r?

Answer 1

a. 1/12=1/x+1/30
30=12*30/X +12
18=360/X
X=20

Answer 2

1/12 = 1/30 + 1/r

The LCD is 60r , so multiply each term by 60 r

60r(1/12) = 60r(1/30) + 60r(1/r)
5r = 2r + 60
3r = 60
r = 20

or

1/12 - 1/30 = 1/r
5/60 - 2/60 = 1/r
3/60 = 1/r
1/20 = 1/r
cross-multiply
r = 20

Answer 3

Thanks to both of you!