Question

A motorboat travels 82 miles in 2 hours going upstream. It travels 106 miles going downstream in the same amount of time. What is the rate of the boat in still water and what is the rate of the current

Answer 1

let's recall d = rt, or namely distance = rate * time.

b = speed of the boat in still water

c = speed of the current

when going Upstream, the boat is not really going "b" fast, is really going slower, is going "b - c", because the current is subtracting speed from it, likewise, when going Downstream the boat is not going "b" fast, is really going faster, is going "b + c", because the current is adding its speed to it.

`\begin{array}{lcccl} &\stackrel{miles}{distance}&\stackrel{mph}{rate}&\stackrel{hours}{time}\\ \cline{2-4}&\\ Upstream&82&b-c&2\\ Downstream&106&b+c&2 \end{array}\hspace{5em} \begin{cases} 82=(b-c)2\\\\ 106=(b+c)2 \end{cases} \\\\[-0.35em] ~\dotfill`

`\stackrel{\textit{using the 1st equation}}{82=(b-c)2}\implies \cfrac{82}{2}=b-c\implies 41=b-c\implies \boxed{41+c=b} \\\\\\ \stackrel{\textit{substituting on the 2nd equation}}{106[~(41+c)+c~]2}\implies \cfrac{106}{2}=41+2c\implies 53=41+2c \\\\\\ 12=2c\implies \cfrac{12}{2}=c\implies \blacksquare~~ 6=c ~~\blacksquare~\hfill \blacksquare~~ \stackrel{41~~ + ~~6}{47=b} ~~\blacksquare`