The resistances 5K and 10 K are parallel, the effective resistance is given by ,
[R1 R2/(R1+R2)] = 5x10/5+10 = 3.33 K Ohm
Since the current through the network is given, we can write (using, V/I = R)
1000/100x10^-3 = 3K + 3.33K + R
From this, R = 10000 - 6.33 ...
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