Answer :
do recall d = rt.
keeping in mind that the boat though it has a speed of "x", when is going with the current is really going faster, if the current has a speed of "y", then the boat is really going downstream at "x + y".
going upstream, is going against the current, and thus its speed is "x - y" then.
now, notice, the time will be in hours, since the rate is also in hours, mph, thus 30 minutes is 0.5 or half an hour, whilst 90 minutes is 1.5 or one hour and a half.
[tex]\bf \begin{array}{lccclll} &\stackrel{miles}{distance}&\stackrel{mph}{rate}&\stackrel{hours}{time}\\ &------&------&------\\ downstream&9&x+y&0.5\\ upstream&9&x-y&1.5 \end{array} \\\\\\ \begin{cases} 9=0.5(x+y)\\ 9=1.5(x-y) \end{cases}[/tex]
keeping in mind that the boat though it has a speed of "x", when is going with the current is really going faster, if the current has a speed of "y", then the boat is really going downstream at "x + y".
going upstream, is going against the current, and thus its speed is "x - y" then.
now, notice, the time will be in hours, since the rate is also in hours, mph, thus 30 minutes is 0.5 or half an hour, whilst 90 minutes is 1.5 or one hour and a half.
[tex]\bf \begin{array}{lccclll} &\stackrel{miles}{distance}&\stackrel{mph}{rate}&\stackrel{hours}{time}\\ &------&------&------\\ downstream&9&x+y&0.5\\ upstream&9&x-y&1.5 \end{array} \\\\\\ \begin{cases} 9=0.5(x+y)\\ 9=1.5(x-y) \end{cases}[/tex]