What is the resistance of a 10 ohm,20 ohm and 100 ohm resistor connected in a parallel circuit?

Ohm's law is R = E/I

Let's say you apply one volt. Then total current is I = 1/R1 + 1/R2 + 1/R3

Total resistance is R = 1/(1/R1 + 1/R2 + 1/R3)

You could use some other voltage if it makes the arithmetic easier. I would use 100 volts for your example. R = 100/(10 + 5 + 1) = 100/16 = 6.25 ohms