[填空题]
The current through t 3)c:/+eo(jmizc joyzk 3a,dl )ba 2s.wu ujrgw0m 68yhe $20-\Omega$ resistor in (Fig. Below) does not change whether the two switches $S_1$ and $S_2$ are both open or both closed. Use this clue to determine the value of the unknown resistance R. $\Omega$
