Question 375325
let 
v>0 speed of the boat in still water, tup, time upstream
 
upstream travel : 80 = (v-6)*tup 
downstream      : 80 = (v+6)*tdown
  
tup+tdown = 80/(v-6)+80/(v+6) = 4
  
 
80(v+6)+80(v-6) = 4(v^2-36)
 
40v = v^2 - 36 => v^2 - 40v - 36 = 0 = (v - 20)^2 - 400 - 36
 
(v-20)^2 = 436 = 218*2 = 109*4
 
v = 20 + 2Sqrt(109) mph