Question 328732
Rate*Time=Distance
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Let V be the speed of the raft in still water and S be the speed of the stream.
1.{{{(V+S)75=30}}}<--- with the stream
2.{{{(V-S)90=30}}}<--- against the stream
From eq.1,
{{{V+S=30/75=2/5}}}
From eq. 2,
{{{V-S=1/3}}}
Subtract the second from the first to eliminate V.
{{{V+S-V+S=2/5-1/3}}}
{{{2S=6/15-5/15}}}
{{{S=2/15}}} km/min
Since there are 60 minutes in 1 hr.
{{{S=(2/15)(60)}}}
{{{highlight(S=8)}}}kph