Answer:
7.5 mph
Explanation:
Let's his speed in still water be v (mph), then his speed in down stream is 1.5 + v and his speed when swimming upstream is v - 1.5
- The time it takes for him to swim 0.9 miles downstream is 0.9/(1.5 + v)
- The time it takes for him to swim 0.6 miles upstream is 0.6/(v - 1.5)
Since it takes him the same amount of time to swim both of these
[tex]\frac{0.9}{1.5 + v} = \frac{0.6}{v - 1.5}[/tex]
[tex]0.9(v - 1.5) = 0.6(v + 1.5)[/tex]
[tex]0.9v - 1.35 = 0.6v + 0.9[/tex]
[tex]0.3v = 0.9 + 1.35 = 2.25[/tex]
[tex]v = 2.25 / 0.3 = 7.5 mph[/tex]
