Missing resistors

[gazdkw82]

for some reason I'm struggling with resistors more than anything else in the electrical science (202/203) course material.

I have a table with 6 examples, each having 3 resistors in parallel and a total resistance. Problem is a lot of the figures are missing and the question asks to find the missing resistances.

I'm not even sure where to start. For example the first has

R1= 120ohms
R2= unknown
R3= unknown
Tr= 48ohms

For resistance 1 i did 1/120 which gave me 0.0083

If I knew the rest I'd devide them all by 1, add all those values up then devide that by 1 and I'd have the total resistance but I'm not sure how to approach missing resistances, especially as no other information (voltage or amps) ) have been given

 

[FatAlan]

Are you able to put the whole question up?

 

[FatAlan]

I'm assuming you're asking 1/Rt=1/R1+1/R2+1/R3 where you know Rt=48 and R1=120?

 

[gazdkw82]

 

[Karin]

reckon that dash means it's only two resistors in parallel, not three. so you only ever got to find one missing, which makes sense for the 202. i just filled a whole page of working trying to solve for two unknowns! sod that..

 

[FatAlan]

I'm confused by the table as some boxes are blank and others have a - in them. Does the - mean there is no resistor? If that is the case you only have the one unknown which is easy to calculate.

 

[gazdkw82]

I think it means it doesn't exist. Iv just tried working out resistance 3 and got 0

 

[Karin]

so for 2a i get R2 = 80Ώ, sounds right i think? not got a calculator here..

 

[gazdkw82]

Yep that's what Iv got with the help from a mate

 

[Leesparkykent]

1/RT= 0.0208

1/R1=0.0083

0.0208-0.0083=0.0125

1/0.0125=80

R2=80 ohms

 

[gazdkw82]

 

[gazdkw82]

 

[gazdkw82]

think iv cracked it