[tex]\begin{gathered} \text{Let;s assume the sp}e\text{d of boat in still water is x.} \\ And\text{ the spe}ed\text{ of stream is y.} \\ For\text{ upstream,} \\ (x-y)\times8=456 \\ x-y=57\ldots(1)_{} \\ (x+y)\times9=783 \\ x+y=87 \\ x=87-y\ldots(2) \\ \text{Now, substitute the value of x in the equation (1).} \\ 87-y-y=57 \\ -2y=57-87 \\ 2y=30 \\ y=15\text{ km/hours (rate of current)} \\ \text{Substitute the value of y in the equation (2).} \\ x=87-15 \\ x=72\text{ km/hours (boat in still water)} \end{gathered}[/tex]
