Question 335696
Rate*Time=Distance
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Let's call the speed of the current C.
When you go downstream, you add the speed of the boat and the current,
1.{{{(15+C)t=12}}}
When you go upstream, you subtract the current from the boat speed,
2.{{{(15-C)t=9}}}
From eq. 1,
{{{t=12/(15+C)}}}
Substitute into eq. 2,
{{{(15-C)*(12/(15+C))=9}}}
{{{12(15-C)=9(15+C)}}}
{{{180-12C=135+9C}}}
{{{-21C=-45}}}
{{{C=45/21=15/7}}} mph