Question 69719
{{{t = d[1]/r[1] + d[2]/r[2]}}}
{{{3 = 12/r[1] + 12/r[2]}}}
s = speed of stream
{{{3 = 12 / (9 - s) + 12 / (9 + s)}}}
multiply both sides by (9+s)(9-s) 
{{{3(9+s)(9-s) = 12(9+s) + 12(9-s)}}}
{{{3(81-s^2) = 108 + 12s + 108 - 12s}}}
{{{243 -3s^2 = 216}}}
{{{3s^2 = 27}}}
{{{s^2 = 9}}}
{{{s = 3}}} km/hr answer