Suppose a box is there which contains a circuit consisting of necessary elements like sources and resistors. Just think about the box only(consider that you don't know anything what the box contains) and you have a voltmeter and an ammeter with you to measure the impedance or take simply equivalent resistance. Is it possible to measure the equivalent resistance with these two only. Remember ammeter acts like short circuit and voltmeter acts like open circuit.

 

Yes, its the thevenin theorem.

Measure the voltage across the terminals with the voltmeter without a load connected to them, then place the ammeter directly across the terminals to measure the current. Finally, divide the measured voltage by the measured current to find the equivalent resistance.