Question 140681
let x="speed in still water" __ d=rt __ t=d/r


upstream __ t+(1/3)=5/(x-6)


downstream __ t=5/(x+6)


substituting __ [5/(x+6)]+(1/3)=5/(x-6) __ multiplying by 3(x+6)(x-6)
__ 15(x-6)+(x+6)(x-6)=15(x+6)


15x-90+x^2-36=15x+90 __ x^2=216 __ x=sqrt(216)