What is the total resistance of a 10-, a 20-, and a 50-ohm resistor connected in parallel?
Why For resistors in parallel you add the reciprocals: 1/R = 1/10 + 1/20 + 1/50 = 0.1 + 0.05 + 0.02 = 0.17 siemens. Inverting that gives R = 1/0.17 = 5.88 ohms, about 5.9 ohms. Note the sanity check: the parallel total is always smaller than the smallest single resistor, which here is 10 ohms.
Watch out The 0.17 ohm choice is the sum of the conductances before you take the reciprocal, and 80 ohms is what you would get by adding the three values as if they were in series.
Parallel total is always less than the smallest resistor. Don't forget the final 1/x keystroke.