Question

 A group of tourists went downstream a river and are supposed to return in 3 hours. How far can the tourists travel down the river if the speed of the current is 2 mph, and the speed of the boat in still water is 18 mph?

Answer 1

they were supposed to return in 3hrs, so down and up the stream is 3hrs total for the whole trip

the distance downstream and upstream is the same


`\bf \begin{array}{lccclll} &distance&rate&time\\ &-----&-----&-----\\ \textit{downstream}&d&20&t\\ \textit{upstream}&d&16&3-t \end{array}\\\\ -----------------------------\\\\ \begin{cases} d=20t\\ d=16(3-t)\\ ----------\\ 20t=16(3-t) \end{cases}\implies 20t=48-16t\implies 36t=48 \\\\\\ t=\cfrac{48}{36}\implies t=\cfrac{4}{3}\impliedby \textit{1hr and 20mins} \\\\\\ \textit{what's`