A source supplies 120 V to the series combination of a 10-\Omega [&10|dash||Ome|&] resistance, a 5-\Omega [&5|dash||Ome|&] resistance, and an unknown resistance Rx.[&R_(x).&] The voltage across the 5-\Omega [&5|dash||Ome|&] resistance is 20 V. Determine the value of the unknown resistance.