Question 964854
r=rate in still water
{{{15km/(r+8km/hr)=9km/(r-8km/hr)}}} Cross multiply.
{{{15km(r-8km/hr)=9km(r+8km/hr)}}
{{{15rkm-120km^2/hr=9rkm+72km^2/hr}}} Subtract 9rkm from each side.
{{{6rkm-120km^2/hr=72km^2/hr}}} Add 120km^2/hr to each side.
{{{6rkm=192km^2/hr}}} Divide each side by 6km.
{{{r=32km/hr}}}
ANSWER: The rate of the boat in still water is 32 km/hr.
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CHECK:
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{{{15km/(32km/hr+8km/hr)=9km/(32km/hr-8km/hr)}}}
{{{15km/(40km/hr)=9km/(24km/hr)}}}
{{{0.375hr=0.375hr}}}