Explanation
Speed of river, $$u$$
Speed of man relative to water,$$v$$
Downward speed of man w.r.t ground, $$v+u$$
Upward speed of man w.r.t ground, $$v-u$$
Downward time, $${{t}_{1}}=\dfrac{s}{v+u}\,\,$$and upward time $${{t}_{2}}=\dfrac{s}{v-u}$$
Average speed = total distance/ total time
Average speed, $${{V}_{av}}=\dfrac{s+s}{\dfrac{s}{v+u}\,+\dfrac{s}{v-u}\,}=\dfrac{{{v}^{2}}-{{u}^{2}}}{v}$$
Hence, average speed is $$\dfrac{{{v}^{2}}-{{u}^{2}}}{v}$$
$$\\12x-6<0\>or\>x<(\frac{1}{2})\\and\>12-3x<0\>or\>x>4\\clearly\>there\>is\>no\>common\>value\>of\>x\\hence\>solution\>set\>will\>be\>a\>null\>set\>\phi.$$
Given,
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