The question uses this figure: http://imgur.com/6tkqRL8
0.967ohm
10 + 15 in parallel gives 6 Ohms
so we have 6 Ohm in parallel with an unknown resistor
However, this network has a total resistance of
V/I = R
5/6 = 0.833 Ohms
1/Rtot = 1/6 + 1/ukn
1/0.833 -1/6 = 1/ukn
(7.20288-1)/6 = 1/ukn
1/ukn=6.20288/6
UK = 6/6.20288
uk = 0.967 Ohms