Astream flows at a rate of 4 mph. a boat travels 70 miles downstream and returns in a total time of 6 hr. what is the speed of the boat in still water?

X - speed of the boat in still water
(x+4) - speed of the boat downstream
(x-4) - speed of the boat upstream
70/(x+4)- time of the traveling downstream 
70/(x-4) - time of the traveling upstream

70/(x+4) + 70/(x-4) = 6
(70x-280+70x+280)/(x-4)(x+4) = 6(x-4)(x+4) /(x-4)(x+4) 

140x = 6(x²-16)
6x²-140x-96 =0
3x² -70x-32=0



x=24(mi/h),
We need only positive value.