Starter

The Ohmmeter shows the correct resistance. Can you calculate it?

You need to group the resistors in pairs or if they're uneven numbers, you need to group two of them and then add the third to the equation. The parallel resistance formula is R1.R2/R1+R2. So for instance with your example you first group 25ohm and 70ohm for instance. 25x70/(25+70)=18.4ohm. Then you use that value and continue the same way with the other resistor. 18,4x85/(18.4+85)=15.1ohm. Once you're at it for long enough you can make rough approximations by knowing the values of the components. If you have a resistor in the ohm range and one in the kohm range, you can pretty much neglect the bigger one etc...

Or you can just do that ((1/25)+(1/70)+(1/85))^-1 this is much easier and faster if u have a lot of resistors @Robert_Kidd @thebugger

Hi Bugzy. I was asking electro9953 and was going to assist if needed. You've kindly explained it to him.

electro9953 - you clearly already know how :-)

To be honest the easiest way to measure it, is to run some current through the parallel resistors and just use ohm's law R=U/I, but if you can't get your hands on a power source, which is pretty much impossible, even a battery will do the trick, you can use the old group equation or the one electro suggested

I focused on pairing them up and it really screwed me in school because I would run out of time on the test. Using electros formula moved me up the food chain.

You run out of time? How many resistors in parallel do they give you, a hundred? Anyway for school purposes you can use the shortened formula, in reality I suggest the fastest method, of running current through them and see what the voltage drop is and use ohm's law. Saves up time and gives a good accuracy too