Two rods having thermal conductivity in the ratio of 5 : 3 and having equal length and equal cross-section are joined face to face. If temperature of free-end of first rod is 100°C and the free end of second rod is 20°C, then calculate the temperature of junction :
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(a) 50°C
(b) 70°C
(c) 85°C
(d) 100°C