You are supplied with a number of 100 Ω resistors. How could you combine some of these resistors to make a 250 Ω resistor?
When two 100 Ω resistors are connected in parallel, the value of the combined resistance becomes 50 Ω. Now to get a resistance of 250 Ω, we must connect two more 100 Ω resistors in series with the above combination. Therefore, the configuration will be two 100 Ω resistors in series with a parallel combination of another two 100 Ω resistors.