Respuesta :
just subtract 252 miles from 12 and 84 from 12 and add those to up
same as the previous one, and again, we'll use "b" for the boat's speed in still water and "c" for the current's rate.
[tex]\bf \begin{array}{lccclll} &\stackrel{miles}{distance}&\stackrel{mph}{rate}&\stackrel{hours}{time}\\ &------&------&------\\ Downstream&252&b+c&12\\ Upstream&252&b-c&84 \end{array} \\\\\\ \begin{cases} 252=12(b+c)\implies \frac{252}{12}=b+c\\ 21=b+c\implies 21-b=\boxed{c}\\ -------------\\ 252=84(b-c)\implies \frac{252}{84}=b-c\\ 3=b-c\\ ----------\\ 3=b-\left( \boxed{21-b} \right) \end{cases} \\\\\\ 3=2b-21\implies 21+3=2b\implies \cfrac{24}{2}=b[/tex]
what's the speed of the current? well, 21 - b = c.
[tex]\bf \begin{array}{lccclll} &\stackrel{miles}{distance}&\stackrel{mph}{rate}&\stackrel{hours}{time}\\ &------&------&------\\ Downstream&252&b+c&12\\ Upstream&252&b-c&84 \end{array} \\\\\\ \begin{cases} 252=12(b+c)\implies \frac{252}{12}=b+c\\ 21=b+c\implies 21-b=\boxed{c}\\ -------------\\ 252=84(b-c)\implies \frac{252}{84}=b-c\\ 3=b-c\\ ----------\\ 3=b-\left( \boxed{21-b} \right) \end{cases} \\\\\\ 3=2b-21\implies 21+3=2b\implies \cfrac{24}{2}=b[/tex]
what's the speed of the current? well, 21 - b = c.
