With three resistors if equal value, there can be the following combinations of resistors:

A. All resistors in series,

With, all resistors in series the combined resistance will be,

B. All resistors in parallel:

With all resistors in parallel, the combination of resistors will provide,

C. Two resistors in parallel and one resistor in series

When two resistors are in parallel, their resistance combined will be,

And when this resistance when combined with the resistor in series we have,

D. Two resistors in series and one resistor in parallel

When two resistors are in series, their resistance combined will be,

And when this resistance when combined with the resistor in parallel we have

Therefore, by combining the resistances we will have a resistances of 10 , 15 , 45 and 20 .